GPU acceleration
All the needed AMD stuff is pretty well documented in the manual. I'm gaming on NixOS with even my "relatively old" AMD GPU.
nixgl
The only time I heard about needing that was when using plain "nix" (not NixOS), as in using Nix under any other OS, to run graphical applications. I haven't touched nixgl at all, and don't see the need for it.
alacritty and kitty
Literally my terminals (kitty is my main) + foot. Both working easily either under main NixOS config or Home-Manager. What's supposed to not be working there?
Literally don't know what your problems are/where they lie. I'm a Linux noob (don't even have the patience for Arch Wiki + install), coming from Pop_OS to NixOS and I'm happy it's all so easy. I've installed it on my main PC and two separate laptops and so happy to just share (basically) the same config on all of them.
Nutze NixOS zu Hause und bei der Arbeit. Solange man die Sprache einigermaßen versteht, ist es ziemlich easy und die Wiki/die Options sind schon verständlich. Meine Kollegen nutzen Bogen und ich kann nichts damit anfangen.