Then when a game is started it starts another Gamescope session which launches the game in a second XWayland session.

No, it doesn’t start another gamescope. It starts a second Xwayland in the same gamescope instance.

The screen brightness adapts automatically to the windows i focus on, which is a good idea.

That’s definitely not something Plasma is doing… Sounds like your monitor is dumb with “adaptive contrast” or just terribly implemented local dimming.

The screencast portal has been around for 7 years. How is it not enough? It is very much GPU-only (if the receiving program supports it, which nearly all do), and encoding the image is up to the app and does not depend on the API.