Looks like a simple zpool remove tank mirror-X worked fine! Wow, I worried about this for nothing 
.
root@nas[~]# zpool remove AllStore mirror-10
root@nas[~]# zpool status AllStore
pool: AllStore
state: ONLINE
remove: Evacuation of mirror in progress since Tue Dec 15 21:40:17 2022
54.9G copied out of 426G at 233M/s, 12.89% done, 0h27m to go