It's much easier than this. Runelite is available on the software center. After installed it will appear on the list of apps in steam when selecting add a non steam game.
Use the steam grid db decky plugin to add art. Make a controller profile or use community made. I've played a bit of osrs this way
Dave the diver is a blast. The premise is simple and fun but it's constantly evolving and building on what's there with a charming cast of characters and events. Id highly recommend it.