Right so your quibble is that part of your audience is not portable, not that federation makes posting harder. You either maintain a centralized service account or you don’t. That hasn’t changed with Meta services. It is Threads that doesn’t allow for portability.

Further a tombstone would point users to the new account if they choose to continue following you. This can be done in a post and your bio.

How bad is it that your two accounts end up in the same feed? By your own admission Threads is a different audience.