I recently switched to NixOS and GNU Guix was also a possible option, and while in retrospective, I can agree with your points, there were two things NixOS does that I want that Guix doesn't offer:

1. Non-free packages (though I guess I could have used Nonguix)
2. An option for Secure Boot

I don't think the assessment "it's badly designed" is fair and your conclusion

Years and years of technical issues plague the project and there seems to be little interest in actually resolving these issues. Guix is comparatively much newer, yet the UX is much better and there are constant improvements in many areas. It also has the advantage of being built from the ground up with a clear design mind."

(emphasis mine) is misleading ; it's not better despite being newer, it's better because it is newer and was able to learn from Nix and improve upon it. Also what would you call the Nix whitepaper if not the design behind Nix?