Replace a Teams guest user or let them have them delegated access to another user

I need to move all Teams data to another tenant. One of the Teams users is a "guest". Systools Teams Migrator cannot copy the data of a guest user because it cannot authenticate the source; the problem is apparently caused by the "Issuer" being an "ExternalAzureAD".

I see three options:

1. Convert the guest user to a member. I have done that in Azure AD, to no avail.
2. Change the "ExternalAzureAD" issuer to my AAD. This is apparently not possible.
3. Create a second user in the source tenant, let this user have full access to the guest user, then delete the guest user. How can I let a Teams user have full/delegated access to another user (not just to the calendar or email, but allow the new user to, for example, post on a Teams channel on behalf of the guest user.

Does anyone have a solution to this?

Thanks!