Five-node #Proxmox cluster? Upgraded from 8 to 9 without issue.
Except a dead Marvell 88SE9230 sata controller card, which degraded the #Ceph cluster for three days, where I borrowed a replacement card from a relatives employer.
Ceph restored to a healthy state in ten-ish minutes, and is happy again.
Now waiting for the actual replacement to arrive, so I can begin my storage migration journey.