I should say that I've worked proffeshionally with ZFS, mdamd/pv/lvm gluster(ugh) lustre(ok) clustered xfs and GPFS.

Along with a bunch of appliances.

mdadm/pv/lvm is about the bottom three. After gluster and ceph.

(For server OS installs that I can recreate trivially, and not for actual data, which lives on a proper SAN because I value my employment)

Do you mean device mapper?

mdadm doesn't need lvm. But if you want snapshots, or to have flexible partitions (anything like the flexibility of zfs) you need lvm.

Also for the longest time redhat used to force lvm on you by default. I assume this was to help with "flexibility"

In practice, on virtual machines, or anything back with decent storage, I'd remove the partition table and just run on the block device directly.

I recommend to try it out with loop devices, play around and build your own opinion about it.