

Have you considered a distributed filesystem such as GlusterFS or DRBD? I believe those support synchronous replication so writes will go to all the configured machines before acknowledging the write. Performance will likely take a hit the greater the number of clusters in the cluster.
Have not tried opencloud yet, but one thing that I find interesting is that it doesn’t need a DB. Plan to test. Currently have nextcloud and find the installation a bit of a pain.