> I'm struggling to understand how something like this can be reproduced consistently across environments. How would you package this inside a Git repo?

Very easily. At the end of the day, quadlets (which are just systemd services) are just text files. You can use something like cloud-init to define all these quadlets and enable them in a single yaml file and do a completely unattended install. I do something similar to cloud-init using Flatcar Linux.