Hello,
Was able to activate the migration for all the existing users and all mailboxes are synced now.
Following "Perform a Google Workspace (formerly G Suite) migration" selectively and activating the migration part worked. However, I did not make use of either subdomain 'gsuite' or 'o365', since I was unable to add any of these to the active users that were already present and had user mailboxes. Furthermore, I did not migrate a "copy" and I did not attempt to merge mail folders, in particular, because these "copy accounts" would have exceeded our licences.
Thank you very much for your support and input! It definitely helps a lot to have someone with knowledge to write to, it is especially helpful in making sense of the system.
Unsure how to combine my comments to a cohesive description of the situation I encountered, so others may get some value from this also? Is it possible to accept your answer and point to the comment section too?
Best wishes!