In terms of aur clones theres some pretty young projects out there, check out MBR (makedeb repository) or Pacstall.
Ive heard of them in passing but have never tried them out tho
Ive barely used Nix, so i cant be of more help than this, but IIRC the .desktop files are created elsewhere. Like inside the nix config folder or something like that
Neovim, and ive been chipping away at learning emacs for a long time now