You can appoint multiple accounts as administrators at Migadu, provided you're not on the Micro plan.

Another option would be the "pattern rewrites" feature of Migadu. It's a sort of "wildcard alias". You, as admin, can define an alias with a asterisk in it (let's say bla.*@) for one of the wife's mailboxes, and she could give out addresses with the asterisk replaced by anything she wants (bla.amazon@, bla.ebay@ etc.)

You can define more than one such wildcard pattern, you could make for her shop.*@, bank.*@, bills.*@, social.*@, and so on, a pattern for every occasion.

She would still not be able to edit these aliases but she would be able to make them up on the fly (as long as they match the fixed part of a wildcard). You can even send from these aliases (you simply use them as your "From" address in the mail client).