That was indeed the issue, there was a dataset that was encrypted and locked. Not sure how it got locked, it was volitile storage for some file transfers. I coundn’t find the encryption key but since it wasn’t storing anything crucial, I deleted it.
After that, the first 500gig mirror set deleted successfully (although it initally said it can’t delete because the pool was in use). When I checked back a few days later, the mirror was gone. I am now in the process of deleting the second 500gig mirror set.
Thanks so much everyone for all the help.