Armored Core 6 is pretty easy to 100% and it’s really enjoyable the whole way there. Just don’t bother with PvP.

And my favorite 100% grinding game that I keep playing just because I enjoy it so much, is the Earth Defense Force series. The game encourages you to play each mission with each of the 4 classes in each of the 5 difficulty levels. Even though clearing Hard also gives you Easy and Normal completion too, you’re still looking at over 200 hours of gameplay, easily. The games have over 100 missions, tons of weapons to gradually improve, and at higher levels become true combat puzzles to solve with said weapons. Cheese is a way of life, in an enjoyable way.