I wondered, Browsers work really well, are already there anyways, have all the GPU stuff etc already dealt with.
It could use the Browsers screencast ability on all platforms, and run with Javascript and WASM.
The stuff could be installed in a local Podman container and thus also work natively on Linux.
Do you know an app that does this, client-side?
I think you can disable most of the toolbars in the main screen if it helps.
You can do that in the “Docks” menu in the topmost bar, unticking any you don’t need.
I think you can freely hide these, maybe more: stats, audio mixer, scene transitions, sources (after you have set up your capture source), scenes.
Then if it’s still a lot, you can untick these in the View menu besides Docks: scene/source list buttons, source toolbar, status bar.
At that point you only have the controls dock, the preview, and the thin top bar.
Don’t forget to reenable the sources dock and the audio mixer if you want to change those settings, though.