>it feels like a huge chunk of specialized operating systems are just someone just putting their own skin over Debian. The vast majority of services and tools they wrap aren't any more complicated than the wrapper.

That's exactly what TrueNAS is these days: it's Debian + OpenZFS + a handy web-based UI + some extra NAS-oriented bits. You can roll your own if you want with just Debian and OpenZFS if you don't mind using the command line for everything, or you can try "Cockpit".

The nice thing about TrueNAS is that all the ZFS management stuff is nicely integrated into the UI, which might not be the case with other UIs, and the whole thing is set up out-of-the-box to do ZFS and only ZFS.