Split brain stuff only happens when you're splitting a single threaded task and put it back together. MDS in CEPH has this problem but that's so far into the weeds here as to be off topic.

Again you're implementing something storage not in storage and taking any storage. Fine if you want to do it that way, but talk about _that_ not hecking ZFS being mah saviour. (Btw daily driving and love that too but an email provider _relying_ on it should raise eye brows)...