1. Expanding Proxmox ZFS Pool

The consensus among ZFS experts depends on your constraints:

• If you have spare bays and older drives: Many recommend starting replacement earlier (around 65% full) with one drive at a time, turning the old drive into a spare. practicalzfs.com This keeps your drives fresher and spreads the replacement timeline.

• If you're space-constrained or prefer simplicity: The double-resilver approach you mentioned works fine, especially if you have solid backups. practicalzfs.com

• If you're concerned about failure risk during resilvering: Adding a new vdev is safer since your existing mirror remains intact during the expansion. Then instead of resilvering you could just reconstruct your VM's on your new expanded ZFS pool from a backup. Then once they are running you can turn off your existing VM's on the old storage and eventually delete them. 

The key consideration ZFS experts emphasize is drive age—mechanical drives typically shouldn't run much longer than a decade regardless of health appearance, and running multiple drives while resilvering increases failure risk.