After trying out everything, this is the controller I use for Celeste! I think I'm one of the only people using hitbox to run Celeste (correct me if I'm wrong)
The 3 leftmost buttons are Left, Down, and Right. The thumb button is Up.
The button just above it which I also use my thumb for is Grab.
The top left action button is Jump
To the right of that is Dash
Second from the right is Jump #2 (for spikejumps, dream block double jumps etc.)
Top Right is Pause (This is super useful for quickly skipping cutscenes)
Bottom Right is Demodash macro I can kinda hit with my palm
This lets me (somewhat) ergonomically use my thumb, index, middle, and ring finger to do mostly everything. I have big hands though so more spacing between them would probably be ideal.
The directions are done with the left buttons so no accidental wrong directions. Feathers take some practice though LOL