Question: How to use `systemd.service` ?

Has anyone thought of a good way to share the same code that creates a `systemd.service` in a NixOS system configuration with a service in a flake? I want to have a NixOS system configuration with many services running across all the systems I manage. Additionally, I want to create a `process-compose` package in my system flake that can set up a development environment with the exact same services.

One idea I had was to rework my services into `writeShellApplication` scripts that get executed by the `systemd.service`. However, this seems laborious and there might be a better way to achieve this.