beyond init and udev script cobbling, I've recently plunged deep into networkd and resolved for the heck of it, discovered networkctl and resolvectl in the process, and it's been a very pleasant experience so far, making easy things trivial and hard things easy.

It really has made things much easier after a slightly frustrating knowledge speed bump.

Unit files seem like a good idea. Does systemd need to do everything it does in order to gain that benefit?

Yes? People don't realize that systemd is a project, and systemd the init system isn't journald or systemd-boot or systemd-resolved. Those are other compatible utilities by the same project. They aren't required to use systemd itself.

Not entirely, for example:

Units can depend on networking managed outside of systemd-networkd using the target -- think of this as a 'run level'

Units can depend on mounts outside of systemd .mount units -- there's a generator that considers fstab

At a certain point, you might want to go further into systemd... but it doesn't require it

Yes. It needs to know what mounts and networks are needed for my various services. It needs to be sure the system is synced with NTP before moving past a certain point. And so on.

Many of the systemd components are separate and optional. You can easily eschew from systemd-timesyncd and systemd-resolved, for example.