If the nix ecosystem moved entirely to flakes, you could just point at the flake in python's repo, pin it to the proper commit hash, and job's done. Might result in a lot of extra near-duplicate dependencies in the store, but that's unlikely to affect you at the level of Python. Otherwise yeah, you're stuck with whatever combinations were blessed by nixpkgs at the time, or with writing your own derivation.
And the error messages are ... well, yeah. I don't find the nix language as awful as some do, but it's still a functional language by and for functional programmers, and being lazy, a lot of errors surface in very non-obvious places. Ultimately Nix could use a declarative config format on top of everything, but I'd rather they ironed out the other issues first. Guix seems to be a bit further along there, but its platform options are more limited.
Tried Mise?
Lots of package combinations didn’t work and I was not skilled enough to figure out why.
The error messages are terrible.
They don’t provide enough versions of packages. I want Python 3.10.4 exactly. But Nix packages by default only provide 3.10.something
I would love to use Nix everywhere, but it’s just too cumbersome for me.