There are exceptions to this such as Proxmox which can actually be added to an existing Debian install. I must admit that when I first encountered it I didn't expect much more than a glorified toy. However it is so much more than that and they do a really good job with the software and the features. If anybody is on the fence about it I recommend giving it a go. If you do, I recommend using the ISO to install, pick ZFS as the filesystem (much much more flexible), and run pbs (proxmox backup server) somewhere (even on the same box as an lxc host with zfs backed dir).

>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.