I'm in essentially the same place. I do wish their marketing pages didn't make such strong claims of privacy - it's an impossible promise. https://www.fastmail.com/fast-private-email/

“We’re more private than Gmail” doesn’t have the same ring even if it’s true.

That's honestly kinda table stakes for anyone attempting to spin up a new email provider. Being slightly better than the incumbent doesn't make it attractive enough to switch for most people.

Well, it's a lot easier to automatically scan and store emails than postcards (eg, flagging all emails with certain keywords or in certain languages)... I guess nowadays they can use OCR to scan postcards, but that would be of doubtful value to any of the 3 letter agencies.

The USPS literally scans and stores all mail sent through the USPS.

And it's a feature, their Informed Delivery service will send those scans to you in an email.

All they get are equivalent to email headers. They don't know what's in the box or envelope unless you've had to declare it with them for some reason.