I see that quota and sieve filtering are on the pipeline, which are two requirements I’d have for switching to a different IMAP server. Do you have any plans for remote authentication support (e.g. via SQL or maybe saslauthd)?
That said, I think I'll find some time to give Stalwart JMAP a try. I've been curious about JMAP for a while, and there are a couple things about Dovecot I'm not too fond of...