Free and Open Source Software

So! Personally, I would recommend against anything that requires you to use their platformed client like Tutanota and Proton. Don't get me wrong, they're both excellent services, but ultimately, they sit antithetical to the point. Making email secure and private and good is hard, bordering on impossible. To accomplish that, you have to jettison core components of email protocols out that stand in the way of making it a secure and private and good messaging service, core components other email vendors will be making heavy use of. This can be fine, as I'm sure you've experienced with your tuta account, most of the time, up to the point where you have to do something weird like "email your parents who don't really understand technology"

Instead, I'd recommend going the complete opposite direction. Find you a vendor like https://mailbox.org or https://posteo.de whose offered services are "We take your money, give you an inbox, and we don't sell your data because that's what we take your money for." After this, there are things you can do to make your email almost secure, almost private, and almost good, but nothing will ever compare to sending messages over a good protocol like Signal Protocol, XMPP, or Matrix (all of which use shared protocols under the hood to ensure secured messaging with forward secrecy).

The bottom line, effectively, for me, is that I want to always be looking for platforms that are making as best use of standards-compliant protocols as possible rather than presenting any form of platform lock-in. It makes it easier to recover from a disaster this way because there will be other platforms with the same protocols available, and interoperability between disparate platforms will be much easier.