I went from Arch to NixOS, so I can offer a bit there.
You definitely won’t want to rely on it until you know a good amount and get comfortable. Things can be made to work, but knowing how to get it done is the main thing most of the time.
Regarding package availability, it’s just a matter of a few oddly esoteric incantations and version controlled code, usually. Binaries are another story but still possible, and python is a special case of that.
It has been an annoyance for me, but I’ve also learned a lot by getting things to work. If you use any niche python stuff you’re bound to run into something. A bunch is already packaged and works fine, though. Either way there’s a bit of extra nuance, which is more to learn.
You don’t have to start with NixOS and can feel it out using nix on any distro. It can be hard to tell if someone will vibe with it. All that said, it could be more than you’re looking to get into, but you can ease into it if you’re interested.
Generally I agree. It feels kind of shoehorned in when desktop is your goal, like more of an afterthought or side effect of the overall focus.
The main thing I hang on to is the code-specified configuration. I never got into managing dotfiles with arch, but that could be a better solution for many people. Especially along with btrfs, numerous containerization options, and whatnot.