Interesting, but my quick search of the Migadu site turned up nothing about support for "tags", formally "keywords" as stored in the Flags Message Attribute of IMAP:
Any email provider would be short of useless without supporting flags from IMAP, as that's what email clients use to see if an email has been read or not, if I'm not mistaken.
AFAIK, Migadu supports most if not everything from IMAP. I'm using tags via Thunderbird and have no issues syncing it between multiple computers and Migadu.
https://www.rfc-editor.org/rfc/rfc3501#section-2.3.2
See discussion elsewhere in the post for difficulties around tags with Fastmail and other providers.