Your best bet is probably figuring out why the graphical session isn’t working and then going from there. Since you’re on NixOS odds are all the logs you need are right there in journald.
Worst case scenario: you might need to pin your system nixpkgs to ~January 2021 until the issue sorts itself out. You can still install newer userland packages if you separately manage them as a flake (this is a common and well-supported pattern in home-manager)
EDIT: found a discussion with good configuration.nixexamples for pinning the system nixpkgs. Once you find a workable pin you could also try inching it up to get a better idea of what broke (January 2021 is a good starting point because it’s the last month before 5.11 released, a newer pin is very likely possible)