I have the same issue using a similar setup with encrypted drives and unionfilesystem.
In OMV4 this setup worked as expected, but in OMV5 the system boots into emergency mode.
The cause for this behaviour seems to be additional configuration options in fstab written only by OMV5.
I was able to work around this issue by manually removing all 'x-systemd.requires=...' options from the pools entry in fstab.
The system then boots in normal mode. After unlocking and mounting all drives the pool seems to work fine. Only the branches tab in Union Fileystem usually shows no drive names which can be resolved by slightly modifying the pools configuration. Unfortunately saving the configuration overwrites fstab with the old settings.
I appreciate the fact that OMV5 now at least checks if all drives are present, but this breaks setups with encrypted drives.
Is there a way to change the default options?
@Final Deleting only these options instead of the whole configuration might make the pool mount again.
I hope this helps