Programming

1. The email stored on T/P is not end-to-end encrypted. T/P have access to it without going through you. Why? Because that's how email works. When receiving mail, their server is contacted by another (non-encrypted) server who says "I have a message for you, accept it and store it". And they have to be able to do that 24/7 without involving you. Same for sending, they have to take your message and store it for several days until they can send it out in clear and another (non-encrypted) server has confirmed it has accepted it. Any pretense that any of this is secure is just security theater.
2. You cannot have multiple clients on multiple devices, because they've replaced standard protocols (IMAP and SMTP) with their proprietary wrappers to maintain the pretense. So you can only use their apps (insofar as they're available for your devices), or their webmail, that know how to speak the proprietary protocols. This locks you into their service.

You can't do secure email. You really can't, sorry. Point (1) above is a game-ending design flaw that makes it impossible, and (2) is just lock-in and hoops to jump through without really adding anything of value.

You could do remote encrypted storage of your email archive but only if you give up the notion that you can also allow that storage server to send and receive messages for you. If they have access then it can be subverted and the whole proposition is worthless.

The way to achieve such storage is by using a remote file storage service reflected locally as a virtual filesystem, preferably with the encryption layer controlled by your device not their service, and use it to store messages managed by your local email client. Your local email client would then use IMAP and SMTP connections to unrelated email servers to send/receive messages. But you'd have to replicate this stack on every device, which is impractical.

The better approach is to self-host your mail archive, with a webmail client on top connected to a SMTP service, and have a local tool on the server that pulls emails from a POP3 server and deletes them afterward. And you can encrypt the disk there if you want, and use whatever you want to access your archive (regular email clients or webmail).