You can run games in Proton without Steam unless that game actually depends on Steam integration. If you just want the Proton compatibility support or whatever.

https://github.com/caverym/proton-caller

That being said, Steam Input can be a useful layer to configure controllers if you use a controller, and that's tied to Steam.

EDIT: Also, that's a generic approach for any Steam games.

Looking online, specifically for RPGM games, it looks like they're internally web-based. You just need a local webserver. I keep a local-only-accessible instance of Apache for running local web-based apps, but it looks like there are people here using the Python mini webserver for something that'll be more-trivial to set up:

https://f95zone.to/threads/easy-way-to-play-rpgm-games-on-ubuntu-linux.93725/

As many of you find out, rpgm games dont work with WINE, i found an easy way to run rpgm games, all you need is a browser & python.

On the command line, navigate to the game’s www folder. then write this on the command line window.

python3 -m http.server --bind 127.0.0.1 8000

then open your favorite web browser and navigate to http://127.0.0.1:8000/

Might be able to open a file directly in the browser too -- I see some people there talking about that -- but that'd let any scripts on the webpage see other files on your machine, so probably just as well to keep it sandboxed to what the webserver can serve. Probably not too much of a concern if this is a game commercially sold on Steam, but a good practice more-generally. I kind of suspect that this may be why Android web browsers don't allow viewing local HTML pages via file:// URLs.

EDIT2: One caveat is that I assume that RPGM probably uses localStorage for saved games, and browsers default to limiting how much a given domain can consume, and if they store a bunch of data, and you play a number of games, it'll probably burn through the cap for 127.0.0.1. Twine games serialize a bunch of JavaScript objects, and definitely can exhaust that. It doesn't look like you can set different storage caps on a per-domain basis for Firefox, though you can increase the global per-domain limit:

https://stackoverflow.com/questions/18530718/increase-size-of-the-localstorage-for-one-domain-name