@woelki I've been working on similar migrations for two of my customers. The migration of linked mailboxes works fine. I know it wasn't supported for a while, but I've not been able to find anything current that says it is unsupported. The only thing I would recommend is making sure your AAD Connect is matching the two accounts correctly and only creating the one account in Office 365. This was the task that took the longest. Configuring AAD Connect to match on SamAccountName and to use the msDSConsistencyGUID seemed to work well (as long as your accounts have the same SamAccountName's in the account and resource forests).
The fun bit comes if you want to decommission the resource forest!