Thanks so much for all these ideas.
1. Connection
Neither USB-C nor HDMI work. I think I found the reason: I have Thunderbolt-connections at my laptop, and the GRUB option nomodeset disables these ports in total (this is why the keyboard and mouse did not work anymore).
2. Install latest kernel
Is this the documentation of what you suggest:
Or are you suggesting compiling it myself?
Maybe you can guide, I really do not know where to start.
3. NVIDIA driver in dom0:
There are quite some web sites with instructions, yet they seem outdated:
(1) https://groups.google.com/g/qubes-users/c/k3gtee7rCUY
(2) Nvidia proprietary driver installation
Unless you have a better idea - I would try to run option (2).
Was this your suggestion?
4. Disable via nouveau.modeset=0
When I tried this, the screen was black and there was nothing I could do other than re-install Qubes. I think this may not work or may require additional work (which I am not aware of).