200 angler quests

Might be a tad cheesy, and it might give you carpal tunnel, but it works.

The set-up

- One angler
- One Journey mode character with the quest fish researched

The process

1. Go to your world, sit in the angler's house.
2. Open the duplication menu, type in "quest."
3. Furiously press the throw button in tandem with clicking on each fish. This will cover the ground in fish, and allow you to pick up whatever fish you finished submitting.
4. Submit a quest fish to the angler.
5. Set time to midnight.
6. Set time to dawn.
7. Submit a quest fish to the angler.
8. Rinse and repeat until you've finished all 200 quests.

Optional: You can leave chests nearby with the reward items he gives, so when your inventory fills up with not fish, you can simply sort them away.
I recommend resting your hand when it begins to cramp.
It took me about 1-2ish hours to get the achievement this way.