You can write arbitrary scripts into systemd... or like one step removed at most? That's not really a difference unless you have some nuance in mind that I don't.

I honestly do not like systemd, either. It is okay for managing processes but I wish it didn't spread into everything else in the machine.

Or if it must, could it actually work cohesively across their concepts? Would be nice to have an obvious and easy way to run Quadlet as its own user to isolate further, would be nice to have systemd-sysusers present in /etc/subuid so they can run containers.

I like what they are doing with atomic distros. It would be great to have a single file declarative setup for something like running a containerized reverse HTTP proxy with an isolated user. Instead of "atomic" but you manually edit files in /etc after install.