My main goal is not to have a reproducible environment but rather an independent one.
I can achieve this by using docker and namely docker-compose where I can describe services I need and start/stop them with ease. That way I can have two versions of the database to be used in two different projects without polluting my global-space with them. That all sounds nice and shiny but I am on macOs and all of that is particularly slow. To the extend when it is even unusable.
As a slight alternative to docker/containers people often propose nix. I like the idea of it. You have reproducible and isolated environments without any virtualizition/containerization on top of it. Cool! I could have said if there was any info on services and how to use them with nix. The only thing I found is that there is such thing as shellHook which allows to do anything like starting a db when you enter nix shell. But you can't automatically stop it if you leave it or if you simply close the terminal.
Is there something in the nix world which helps manage services with the ease it helps to manage libraries/languages/frameworks?
Sounds like you need a process manager.
You might be able to use nix-processmgmt to write service configurations that you can run with supervisord on macOS. Launchd is also supported, but project configuration shouldn't be mixed with system configuration, if avoidable.
I haven't tried this yet, because I can do my backend work on NixOS with arion, which uses docker compose as a backend. I'll be interested to know what you think of nix-processmgmt.