Clip Studio Paint: it goes on sale about every 3 months or so (next one should be around March), EX version (the one not limited to 24 frames; yes, that’s right - 24 frames total, not 24 frames per second) will cost you $129 while on sale. I bought it in 2015 when it had no animation tools and still getting free updates.
If you’re into japanese style animation, you’ll find a lot of information and videos about using CSP in anime pipeline (kinda useless for game vfx, but general CSP setup and tips can be useful).
There’s also TVPaint, but the price is too high if you ask me.
For something less known: Plastic animation paper (old version is free, beta is free), old-school DigiCel FlipBook, CACANi with automatic in-betweening, and a bunch of open source tools like Enve and Pencil2D.
https://www.youtube.com/watch?v=AazeNjdZibI
Plus take a look at Keyframe Pro for studying reference material.