I did try GeForce Now, since it has BeamNG. From what I could tell, it was about the same as Paperspace, except I didn't have access to the underlying OS so I couldn't install mods outside of Steam. It doesn't have X-Plane. It's slightly cheaper and easier to use since you don't have to manage the lifecycle of a host.

And Stadia doesn't have either game.

If you run your own host, you can play whatever game you want, and mod it however you feel like. Otherwise, you're stuck with whatever curated stuff the platform provides.