“do one thing well”
Arguably, Systemd does exactly that: orchestrate the parallel starting of services, and do it well.
The problem with init.d and sys.v is they were not designed for multi-core systems where multiple services can start at once, and had no concept of which service depended on which, other than a lineal “this before that”. Over the years, they got extended with very dirty hacks and tons of support functions that were not consistent between distributions, and still barely functional.
Systemd cleaned all of that up, added parallel starting taking into account service dependencies, which meant adding an enhanced journaling system to pull status responses from multiple services at once, same for pulling device updates, and security and isolation configs.
It’s really the minimum that can be done (well) for a parallel start system.
Debian server minimal, is kind of a “core Debian”. There are netinst versions that can be even smaller. The Debian base image for Docker is even smaller than all that.
There is also an Ubuntu minimal install that you could call “core Ubuntu”.
But more importantly, and I can’t stress this enough: YOU CAN SWITCH DISTROS WITHOUT REINSTALLING. Might need to do some cleanup afterwards, but it’s perfectly doable, more so between Debian based ones.