Honestly, Guix moving to Codeberg sounds like the best option for now. As a Nix-like (as in the Nix package manager) package manager/distribution, the code forge has to be convenient for large code bases (tooling source code and package definitions) in addition to being easy to use for new package maintainers.

NixOS has done this with Github so Codeberg seems like the fitting alternative for Guix (and Guix System)