Data which is sensitive that needs to be stored securely needs to be kept on a computer that is OFF-line.
Basically, user a "POP" email address, have it downloaded to your computer THEN back it up to some other machine.
No email provider is the "Fix-all solution" they make themselves out to be. My computer-geek friend (more so than I) suggests that all email is encrypted, and will still be spied upon by the relevant spy agencies whether you think they are or not. And even if they are "safe," I imagine the relevant agencies might get upset over such a service...
For some group that insists on your own privacy so much, I am concerned this "ProtonMail" company has at least 5 tracking bots following my every move on their site.
And they make you pay - so not only are they harvesting your personal data and selling it to some firms that we have no clue of (Which IS THEFT!), but they are making you pay some servicing fee too.