A lot of the 'hardness' of the goals depends on player count IMO. Generally speaking, I found most goals easier to achieve with 4 players than with 1 or 2, but there are some exceptions to that rule.
Easy:
-Make Friends. On average you need these anyway just to reveal the CC bundles, and a decently sized friend pool allows for you to get the maximum out of gift abilities. Consistently easy.
-Save enough Gold. Gold is fairly abundant lategame even if there is no farmer or fisher among yourselves. Gets more trivial the more players you have.
-Expand the Farm. Essentially the goal reads: 10 gold per player plus some wood and Robin visits. The buildings are generally useful to have around though and the wood is fairly easy to get while doing some river fishing or buying animals. Also gets more trivial the more players you have.
Medium:
-Raise Animals. This requires some more serious cash, but cash is the one resource that's fairly consistent to get. Again gets easier the more players you have.
-Upgrade Starting Tools. Almost trivial in a 1-2 player game if you have either the pickaye or the fishing rod, but you can get screwed out of ore. Harder in a 3-4 player game as you suddenly need a lot of ore, which you can get screwed out of.
Hard:
-Catch Legendary Fish. Functionally requires the fishing profession to clear the fish track fast enough. This can be especially rough in a 1-2 player game if the legends don't show up decently early; 3-4 players have an easier time here.
-Explore the Mine. Functionally requires a pickaxe with a few upgrades and can get absolutely scuffed by a mine level with empty HH/HJ slots. 1-2 players can potentially work around these levels a bit better due to needing less stones for staircases; 3-4 players functionally require a very dedicated miner.
-Restore the Museum. Decently easy for 1-2 players especially with the Foraging profession; you can essentially donate whatever you find. Hardest goal by far for 4 players IMO; there's basically no consistent way to get artifacts or minerals (the closest being a high pickaxe level miner on level 7+, but it's easy to miss geodes on these levels) AND you basically have to have locked this goal one week before the end whereas any other goal you can easily finish off in the last week.
My hardest set of goals would be something like this (ordered by hardness):
4 players:
- Donate to the Museum
- Explore the Mine
- Catch Legendary Fish
- Upgrade Starting Tools
3 players:
- Explore the Mine
- Catch Legendary Fish
- Raise Animals
- Donate to the Museum / Upgrade Starting Tools
2 players:
- Catch Legendary Fish
- Explore the Mine
- Raise Animals
- Donate to the Museum / Upgrade Starting Tools
1 player:
- Catch Legendary Fish
- Raise Animals
- Explore the Mine / Upgrade Starting Tools
- Save enough Gold / Upgrade Starting Tools