Good news, I was able to brtfs restore onto a spare hard drive I have, and I unassigned the pool devices, erased one and copied the files onto there, boots right up. I kept the second pool device as backup. I didn't try the first option since I already started the second.
I made it into a single drive cache, with xfs and I'll probably do very frequent backups of it. I've read that others have issues with btrfs as well...