Unfortunately I think you probably will have to work on it if you want to not break existing configurations, so maybe NixOS is not for you. You could try alpine possibly? But with that you will have to do the same with init.d, so it's going to be somewhat of an uphill battle either way I guess. This is generally why I would discourage maintaining a service manager outside of an established distro, I tried to do it for about the same amount of time as you and it became not worth it. Upstart in particular has been basically entirely replaced by systemd.

I already use upstart on void no problem. Alpine would be a similar deal with me just managing the configs as raw text files via rsync. In fact my fork is already packaged in Alpine. A few people use it as a session manager for sway.

There is no chance that my Nix changes would affect existing configurations. They would be completely self contained.

I mean if you want to run existing configurations on your setup. Those aren't going to work. You can just install Nix on void though.

I had some users too but it still wasn't really worth it. I can't suggest it as a business model or a hobby project, you are basically going to be highly constrained by everything else on the system and your setup will just start looking like everything else eventually. Systemd also works fine as a session manager for whatever.