SOLVED: Adding _netdev to the options in /etc/fstab appears to give ZFS and/or fuse (not sure what service or module was not loading quickly enough) enough time to be ready to receive the mount.
Hey 
I recently upgraded my system from 5 to 6 and faced the same issue of a read-only root fs. I was really confused how that could happen and tried many things from fixing services and fstab configurations to running fsck etc. till I stumbled upon this thread and read your way how to fix that. I tested this asap adding the _netdev option to the mergerfs mount in fstab and yeah, problem solved! So happy about that and so much thanks to you! Not sure if this could affect others but maybe someone should investigate this. Have a great day!