Filebackup of the host. Especially /etc/pve for pve specific configuration, large part of it is handled by cluster, but something is node dependent (example: certificates).
Search forum at least.
Running PVE 9 on DL360/380 Gen 8-10 without problem.
You can install rsyslog, attach monitor for kernel dump, maybe there will be something not in journal.
It's standard debian anyway.
Other thing - firmware, power, etc etc etc.
Without upgrading the 2nd SAN node:
What will happen, when you shutdown the 2nd SAN node?
What will happen, when you shutdown the 1st SAN node?
I think, it's SAN support thing mainly, because with 2nd SAN node upgrading it blocks something...