This is a great question - and I'm sorry there are no replies. I too am migrating from OMV4 to OMV5 and have over a year of Duplicati backups to B2.
To provide more detail - I have an OMV4 server and an OMV5 server. My intention is to take the data drives out of box 1 and insert them into box 2 because in both cases the OS is running on an isolated SSD.
Is there a way to migrate the DB that is foundational to Duplicati from one container to another?