I got into this issue recently, and the ones on F droid are bad like you say. So after trying many I ended up using 2 apps. If you want an open sources app, then good old Ghost Commander works very good. It has very good transfer rate and stability. The downside is the old interface, although some people might like it, and that it cannot keep the connection on the background. So if you lock your phone or close the app, the transfer will stop. The second one is not open source, but it works really well. It’s CX File Explorer. It has a fast transfer and can keep working in the background. All the other file managers that I tried had connectivity issues and would not complete the transfer on big files.

You are probably looking for something like games on whales https://games-on-whales.github.io/. If you have an extra weak gpu or an integrated one besides your 9070xt, you can use that for the video transcoding and the big gpu for the game rendering

Besides the time and responsibility of running the service, your biggest cost is going to be storage and maybe even transfer. None of the cheap VPS have big storage or bandwith, so I would not offer a service like this if I were you.