I’d recommend an e-mail provider that supports custom domains, so you can easily switch at any time. I personally use Purelymail; I’ve also heard good things about Fastmail and Migadu.

I've never quite finished moving all my domains to Proton from google, but I think I'm leaning towards infomaniak.com for now, on the Business plan with my and my brothers family using the service

Tuta mail and calendar has been a real easy switch. I imported my emails from proton and just started a fresh calendar because I could and it was a good excuse to clean mine up.

The iOS apps work well and the experimental flatpak on desktop has been totally fine.

I have very little complaints so far tbh. I’m not like blown away or anything but everything just seems to work.