I've bounced off Nix a couple of times and figured that the problem is that it was too different. A lot of Linux distros (especially post-systemd) have a fair number of similarities and it's pretty easy to translate knowledge between them. NixOS _probably_ works very similarly, but it's also got this additional declarative configuration layer on top, which is the point of using NixOS, and it's got its own special way of doing things.
You can't translate Linux knowledge to a NixOS config directly, you need NixOS' documentation to understand the "NixOS Way" of doing things, and the NixOS way of doing things does not translate into other distros. And the documentation is difficult to understand and at times seemingly arbitrary.
You have to commit to learning NixOS and understand that you are only learning NixOS, it won't help you use anything else. Other Linux distros help you learn Linux in general and inform your understanding of Linux system construction, but NixOS is its own thing. Plenty of people learn NixOS anyway, and more power to them, but it's definitely pushed me away.