You could also just run IMAP/JMAP/SMTP as separate components, I can’t see any place in the Stalwart documentation - or in the Docker image itself - where monolith is the only option.

I haven’t tested the setup myself yet, but me and another root are planning on testing a setup of Stalwart to replace a semi-broken IMAP/JMAP setup for a computer club, keeping the SMTP as is.