I'm in the same boat. I think I'm likely going to stand up a Nextcloud instance on a VPS for calendar/contacts/docs and move mail (somewhere). Paying by user/mailbox is really not a good deal for the way we've been using it, so it will either be someone that charges based on domain/traffic/maibox size or something self hosted for received mail with a pay-per-message smtp smarthost.

It's really not something I'm happy about needing to figure out all of a sudden.