Is there a way to install either Debian 12 (or 13) or Proxmox on Ceph root?
Ceph itself is a distributed storage cluster , not a local disk formatter like EXT4, ZFS or XFS. When you format a drive with something like EXT4, the kernel talks directly to the sectors on that disk. With Ceph, there is a massive layer of software (RADOS) between your data and the disk.
I understand that it is not a local filesystem, but if you install, for example, the ceph-mgr-dashboard, I think that there is also a way to install, again, for example, iSCSI on Ceph.
Also keep in mind that ZFS on Linux is also a massive layer of software between data and disk as well (given the capabilities of ZFS).
This way, you get all of the benefits of Ceph as well as the benefits of Proxmox/Debian.