Just tried again to switch from stable to testing with my GTX 1070 setup. Tried to downgrade the driver due to the resolution issues as @soundofthunder suggested.
But I can’t install any other Nvidia driver version, as they conflict with video-nvidia, nor can I remove video-nvidia as it is required by linux617-nvidia. Tried the same with kernel 6.18, same conflicts.
Update: After a reboot, the output of conflicting packages changed. Somehow some packages related to Virtualbox where causing issues. Also Zed had a direct dependency to the the Nvidia vulkan package.
After removing all those packages, I was able to downgrade to Nvidia 575 and for now my screen is back at 2560x1080.