Hi,
I also had issue with the font change in 25.05
What solved it for me was to be on 24.11, then do a nixos-rebuild boot instead of nixos-rebuild switch
Other solution that might work for you is to do sudo fc-cache -rv + reboot/logout
After looking at your repo, I only see one thing that seems weird:
Why are you getting all nerd-fonts and not only the one you use?
Here is my home-manager config in comparison: nixconf/user/alex/home/font/default.nix at 4356803c8499001cbb2d419114d3c6f9f3d2043d · Alexdelia/nixconf · GitHub
Here is my merge commit with all changes between 24.11 and 25.05 Merge pull request #5 from Alexdelia/25.05 · Alexdelia/nixconf@4356803 · GitHub