Your goals for later years depend not only on what you've accomplished so far, but on what you enjoy doing in this game. Try to arrange things so you can do more of what you enjoy. It's hard if not impossible to pursue all the late game achievements at once - go for your favourites.
If you like milking cows, then consider building more barns and setting fences so you can keep track of the animals. If you like going mining, then you want automated sprinklers and pigs - or possibly no barn at all. If watering your crops gives you satisfaction, then you won't be so keen on the sprinklers; if harvesting your crops is the good bit then you may not be interested in a Junimo hut. If you like fishing, the Master Angler achievement may be the one to aim for.
Setting up sprinklers and other elements of automation is generally going to help you make lots of money while still having free time to go around socializing or fishing or courting. This is also a good path if you want diversity in your gameplay, as it means you have time free from farm tending to turn to whichever other endeavour suits your mood. (You'll still have to plant and harvest some of the time.)
Goals you are likely to pursue in the later years are:
- Mystery of the Stardrops: Collecting all Stardrops
- getting a supremely happy spouse (which means getting married)
- A Complete Collection (donating every possible thing to the museum)
- Master Angler (catching every possible fish)
- PolyCulture: shipping 15 of everything.
- Legend: Earning 10 million.
- The Beloved Farmer: getting a 10 heart friendship with 8 people.
- Gourmet Chef: cook every recipe.
- Craft Master: craft every item.
- A Big Help: complete 40 'help wanted' requests.
- Full House: married with two kids
- Master of the five ways: max out all 5 skills
- Protector of the Valley: complete all monster slayer goals.
You may also be interested in getting the special dwellings from the Wizard: the Junimo Hut (will harvest crops for you), the Gold Clock (stops mess and decay), the Earth Obelisk (teleporter to the mountains) and the Water Obelisk (teleporter to the beach). The Return Scepter, available from Krobus in the Sewers, is another 'make my life easier' Item of Exceptional Cost. Most of these require, primarily, a million dollars or more; they also require other resources in modest amounts, which can usually be collected over a few days. The Junimo Hut is a notable exception, costing merely 20K; however it also needs 200 stone, 100 Fibre (which is not much used elsewhere, and it's not usually available in much quantity. So it's worth saving as you go along.) and 9 Starfruit, which is still scarce in year 2 but you should be able to grow a lot of it in year 3 or later.
For most of these objectives, you want to develop your farm so that it makes lots of money with a minimum of effort. The more automation, the better! So we're going to take the strategies mentioned for Year 2, and we're going to turn them up to Eleven.
If you have the Statue of Perfection from Grandpa's shrine, you can kick back and relax to a certain extent; you'll average 5 iridium ore per day, which is enough to put out one new Iridium Sprinkler each day. Even if you've got 40 batteries cached from year 2, it won't take too long to use them all. (80 Sprinklers will probably do the whole farm, and it's hard to plant that much in the vanilla game.)
Or you can keep hitting the Skull Cavern for more iridium. The big trap with the Skull Cavern is that you have to start from level 1 every time you go there. For best results you want to be catching the bus at 10am, as soon as Pam gets there; and staying in the Skull Cavern until you pass out at 2am. For a standard run, you want the best sword you can get (Lava Katana will do nicely) and plenty of food; a Gold pickaxe at very least and an Iridium one if you can afford it. And you go down as fast as you can. Only postpone going down a stair or a shaft if you've got ore in sight. Kill the purple slimes, as they are more likely to drop iridum ore (and even iridium bars sometimes) than you are to actually find lumps of ore. You're doing nicely to get to level 30 and very well if you get to level 50.
A Mega run to Skull Cavern takes a lot of preparation but can yield more Iridium than you know what to do with. You want a big stack of high health food (Pink Cake is recommended, for which you will need to grow a lot of Melons.) You want a big stack of bombs (Mega-Bombs, for which you will need to collect a lot of Solar Essence and Void Essence.) Miner's Treats are good because of the bonus they provide to your magnetism. And you want an absolute mountain of stone - enough to fill about 30 stacks of your inventory. You can buy the stone from Robin, for about 600K (Use shift-click to buy it in larger quantities.)
Use the stone to craft staircases and plummet through the levels to at least level 200. (Crafting does not take game-time.) Don't fight monsters. If they fight you, just go down another level and eat your health back to full. Don't walk around; if you don't like what you see, use another staircase. Don't use your pickaxe on Iridium nodes. Once you're in an Iridium Heavy area, let off a Mega Bomb; collect the results, and if a shaft is uncovered, use it; otherwise place another staircase. Repeat until 2 am. This should allow you to collect ludicrous amounts of Iridium.
If you're pretty comfortable with the placement of your iridium sprinklers, you may like to place them on stone paving. This means you don't need to be careful to hoe around them; you can hoe 6x3 over the top of the sprinkler, and because of the paving, the sprinkler won't pop up. (If you missed this and you do hoe up a sprinkler, hit the desired spot on the ground with the pickaxe to un-hoe it; plonk down a square of paving and then put the sprinkler on top. Also works with Scarecrows.)
Repeating crops, like Berries and Coffee, are one of the better ways to turn soil and water into money. Coffee is particularly good in that it will normally yield 60gp worth of beans every 2 days. But harvesting all of that is time consuming. Once you have completed the Wizard's quest, you can purchase Junimo Huts, for 20K + fibre, stone and starfruit. A Junimo Hut will harvest ripe crops in a 17x17 square on your farm - you just have to collect the crops from the hut afterwards. As automation goes, this is wonderful!
On the standard farm, a simple way of ensuring good use of land for Junimos is to arrange 8 iridium sprinklers in a 3x3 square (of 5x5 squares) with a path through the middle. The Junimo hut can then occupy the centre of this square and the Junimos can be counted on to get to every crop on the 8 sprinklers. This doesn't get optimal use out of every possible square, but it's pretty good.
Junimos are also happy to harvest regular crops, but you'll have to plant new seeds afterwards. As of version 1.2 there is nothing to automate the planting process for you.
Lots of Kegs
The best profits are from Wine, and wine needs Kegs, and Kegs need oak resin. Have you got those Tappers going?
In fact, it's probably time to plant some Oak trees to help your Oak Resin supply. Surprisingly, you can plant trees off the farm. The Train Station is a good spot for this. Take all the acorns you have (I hope you haven't turned them all into Field Rations) and all the acorns you can get by shaking your local oak trees (empty hand and right-click on the trunk) and go up to the Train Station; hoe the dirt and plant the acorns in the hoed patch. These trees don't need as big a spacing as fruit trees; instead of planting one every third square you can plant them every second square. They still need one clear square all around. You'll need to keep collecting wood elsewhere, so that you can make the tappers and the kegs. (This means clear-felling Cindersap Forest every season - and planting the pinecones if you can, to improve the amount that grows back.)
Kegs also need Copper and Iron. These ores are plentifully available in the Skull Cavern; you should collect a lot of them in a normal run. If, however, collecting these ores seems like a poor use of your time, you can instead simply buy lots of ore from Clint the Blacksmith; in which case you will also want to buy coal.
The next question is where you're going to put lots of kegs. The Shed is what you're expected to use for the job; a building that takes up a little bit of your farm and provides a lot of room inside. However, some people feel that an empty (Deluxe) Barn provides a better tradeoff of land space to internal space.
Wherever you put them, you will need to arrange things so that you can get to all the kegs. So when you're filling a building or confined area, usually you put kegs all around the perimeter, and double rows of kegs through the middle. You can arrange kegs in little U shapes, but this doesn't improve the basic 2-1 keg to walkway ratio and it's fiddly.
The less obvious solution is to put kegs off your farm. You'll need to avoid spots that villagers pass through, as they destroy items in their path. If you search "villager pathing" you'll find maps that illustrate where they do and don't walk ... or you can avoid busy areas and place your kegs where few people go anyway. Most of the Bus Stop is good for this (only Pam and Abigail go to the bus); also the Train Station (noting that Linus likes to visit the Bathhouse); the area near the Adventurer's Guild, and the Quarry. Placing kegs in the Quarry will prevent stones spawning where the kegs are; however, the loss of ore is negligible by the time you're that wealthy, and absolutely no villagers ever go there. The Rock Pools might be another good place to put Kegs, but the pools get in the way and coral won't spawn where you have kegs.
Use Indicator kegs - placed where you can readily see them, and filled last after the shed, barn, or region to which they correspond. When they're ready, it's time to go empty all the others.
Use the Cellar. This addition to your house enables you to age wine to Iridium Quality, doubling its value. It takes two seasons - but as effort-for-money goes, that's a real win! You'll want to move some of the initial casks so that you can pack things in better - there's only ever one Cellar. You'll also want more hardwood and regular wood to craft more casks. Initially you might load casks with Pomegranate or Peach wine, but in the longer term you'll only age the best wine - Starfruit wine or Ancient Fruit wine.
It's possible to fill the cellar right up, but then you have to dig out casks to get to the ones behind them. If this is your plan, your best option might be to fill one row at a time - then when the row is ripe, you can dig up the line, without risking digging out partly aged casks.
High value, low effort crops
Where you are planting crops, and particularly where you are harvesting them yourself, you want the crops that require a minimum of effort from you. At the beginning of the game the rapid turnaround crops were the go; now you want the slow ones. Each season features seeds that take 13 days to grow - Cauliflower, Melons, and Pumpkins; these are still very good. Come Summer you should be able to plant hundreds of StarFruit (seeds from the Oasis in the Desert); these make very valuable wine, which is worth aging in the Cellar.
In the Greenhouse you should be able to grow lots of Ancient Fruit, by reinvesting your fruit using seedmakers until the greenhouse is full or you decide that's as much space as you're prepared to devote to one thing.
Don't sweat the small stuff
Your time becomes quite valuable and it's time to give the villagers some business. Don't spend very long collecting stuff that you can buy cheaply.
Forget harvesting your own hay. You've got better use for that land and better uses for your time. Buy a mountain of hay from Marnie and put it in a chest near your Silo or inside your Barn.
If you don't want to mine for Copper, Iron, or Coal, don't. Clint will sell it to you. Gold costs a little more. Unfortunately Iridium is strictly your problem.
If you can't get enough wood or stone, see Robin for it. I don't know where she gets it all, but she can sell you 30K wood or 30K stone if you've got the coin to pay for it. (Unfortunately there's no Hardwood shortcut and it's very useful stuff. You'll have to keep going back to the Secret Woods.)
Willy sells bait for 5gp per bit. You don't need to hunt your own bug meat. (If you're visiting the mine levels 20-30 for Earth Crystals, that's a different story.) (Worm Bins are another way to solve this problem.)
Collapsing at 2am only sets you back 1000 gp, and a bit of food to restore your energy in the morning. Go ahead; stay up late! (If you're inside your house tending casks or similar when 2am rolls around, it'll only cost you the food.)