Would that require giving up the access credentials to app-supporting servers or there’s a way to avoid this and make two servers meet “in the middle” with client orchestrating the initial setup (at least with JMAP)?

I self-host and don’t like how way too many client apps deal with credentials in the name of convenience, many without even announcing that they do send them somewhere outside the device. I don’t currently have JMAP but I’m interested in replacing IMAPv4 as soon as something better is ready aka has all the features I use and mobile and desktop clients (I don’t really care about the webapps).