Tor package in nixpkgs is okayish maintained, although they sometimes take over 48h to merge a patch, which is often unacceptable especially if there has been a security vulnerability and the directory authorities refuse this relay anyways.
The Arti package sometimes misses a version, there hasn't been 1.8.0 for example.
oniux is well-maintained though, probably because cve knows the maintainer personally.
The Tor service module is very torrc centric and does not provide an easy way for running multiple relays on the same machine.
Consider offering a flake with the recent version of ctor, hosted in a git repository on gitlab.torproject.org maintained by the Tor community, although given that updates only involve a torrc change, TPA may do so too?
Provide a custom services wrapper in that file utilizing NixOS containers for running multiple relays. Make them as stateless as possible, i.e. only storing/mounting the rsa, ed25519 and ntor secret keys.
Clara asked on tor-relays@ what folks use, there has been useful feedback: