That’s kind of a weird thing to try, using controller for Aseprite.
Have you tried binding the functions you want via Autohotkey and running the script every time you launch Aseprite? You could even make a custom batch file that starts the script for you, launches Aseprite, applies the hotkeys only when Aseprite window is in focus, and ends the script when you exit the program. It’s tricky, but you could get it to work.
Sadly I’m not familiar with Steam controllers and such stuff, so I can’t help you there, I’d do it with Autohotkey.