@LiftedLorax @Tutanota the practicals of it were simple. Exporting calendar, and emails. Importing on the other side. Migrating DNS wasn't difficult either. Just remember to remove those domains from proton after sending and delivery is working on the new side, otherwise you'll get daily panic notices from proton.
Contacts doesn't have an export on the proton side, but tuta has an import via their mobile app which got everything I needed. I ran a contact merge after that to clean things up and it just took a few minutes.
Cancelling your plan with proton will require an email to their team as there's no web option. That took me 3 messages total. The first got a reply with their same PR talking points about the recent FBI compliance event. Then i had to tell them I wanted to proceed twice more before they did it.