SUMMARY I installed the official 570 driver from nvidia and now kwin disabled all desktop effects while compositing (transparency) still seems to be active. I thought I restart kwin with kwin_x11 --replace and got the error: failed to load driver: nouveau kwin_platform_x11_standalone: glSwapInterval is unsupported which is probably the cause for desktop effects and so on being disabled. no nouveau module is loaded and nvidia-settings shows that the nvidia driver is used.
KWin isn't responsible for which driver(s) to load; if glvnd doesn't load the proprietary driver, that's caused by some system misconfiguration (which you can ask help with in your distribution's support channels)
Okay, the problem is that something is horribly broken when using the 570 driver. The renderer is llvmpipe, thats why the effects are disabled and the transparency effects have no blur. When I delete the xorg.conf created by nvidia-settings, the renderer becomes "zink Vulkan 1.4(NVIDIA GeForce RTX 3060 (NVIDIA_PROPRIETARY))" and I get a black screen with compositing enabled. I can disable compositing and work without GL, but in the end it is worse than llvmpipe. What renderer should kwin be using in the best case? I think Vulkan sounds like it's what KDE should be using? But why am I getting a black screen (or when switching it on again after having it turned off black windows but visible titlebars)? I'd also like to finally give you some feedback on the Wayland questions that were waiting on me having newer drivers, but it seems that it also chooses llvmpipe instead of the nvidia driver. The loaded module is nvidia, nouveau is blacklisted and I didn't configure llvmpipe anywhere (I think it is probably a fallback options?).
I must agree with Torvalds on Nvidia. After a lot of different driver versions I used the latest runfile from NVIDIA instead of trying Debian packages (I hope it won't break things that Debian expects too much) and now it works. I also got the first time Wayland to run without absolute show stoppers. For the record: glxserver_nvidia could not be loaded. It was present but Xorg did not load it no matter how I configured the paths, created symlinks and so on. Probably similar to https://bugs.archlinux.org/task/70287 and some related reports, but none of the solutions anyone suggested there fixed the problem. Someone suggested that the upstream .deb packages are not as complete as the runfiles.