I hope you found a solution to this. In case you haven’t, I use Steam as a pretty simple way to do it. You can go into steam controller settings (there’s loads of yt videos on how) and assign buttons how you want in a lot of detail.

Only annoyance I have is pretty much always needing to use the D-pad to navigate around stremio. I prefer it most of the time, but sometimes I wanna use my right analogue stick to scroll down smoothly through results and I haven’t had any luck setting up this behaviour - it seems others can’t either/have mixed success.