I only have one minor complaint about Fastmail after 5 years. I can’t do a full-page screenshot of a long email on my iPhone.

Have you tried requesting it? As a former developer there, I would expect you to get a response of “sorry, we didn’t know about that because none of us use it and no one has requested it before, fix incoming” or “yeah, unfortunately in our current architecture it’s not possible to do this because of [some specific reason]”.