Do you know if XFS can shrink volumes yet? As far as I'm aware, that's the only limitation it has compared to other filesystems of that era.

It can't, but you're probably better off using trim/discard/virt-sparsify rather than shrinking filesystems. Even on filesystems like ext4 that support it, shrinking can cause strange fs performance problems.

Nope, still can't. Not online, not offline.