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.
If nothing has worked for you so far, and you have an iPad, I would really recommend Procreate. It’s cheap, no subscription cost, and it supports animation pretty decently. It’s also an excellent illustration tool.
There are already great software mentions (I can’t even add another rn). But I wanted to chip in on how to approach this list:
In my experience it is best to just take a look at each software (some are free or at least have a trial) and use the one that “makes click”. There is no right or wrong imo as all have their pro and cons.
I personally use CSP and Krita. Depending on what I want to do I take which one I am more comfortable/quicker with for the task. But both actually do not have a sprite sheet exporter built in (at least right now).
For CSP I also use Free Texture Packer after exporting each frame of the animation.
For Krita there is a plugin called kritaSpritesheetManager.
I dipped my toes in OpenToonz and there where some things to set up before hand, but it was also really nice to use. But I stuck with the before mentioned programs.
I hope you find something that resonates with you!