I've now switched from Proton Mail to Tutanota, taking advantage of their new basic plan that allows for 3 custom domains. Proton's Mail Plus plan only allows for one, and to add more you need to upgrade to the Unlimited plan, which for me is ridiculously more expensive when I only want their email service. But I've been pretty happy with their service otherwise.
Except for their web client, which for me was always slow and therefore a pain to use. Tutanota's desktop client is so much smoother, weirdly so, although it remains to be seen if it will be able to handle it after I import my several thousand emails to it, ha!
Note that Tutanota has fewer features than Proton Mail. This might not be acceptable to people who are used to scheduled send, more fine-grained inbox rules, etc.