I am in the same boat.
Discourse has IMAP support but only for group inboxes, not for "reply by email".
Discourse was one of the big apps I want to be running on Cloudron, and having to add an external email provider just for POP3 really sucks.
Discourse has their "mail receiver" docker container that runs all of this for you if you self install elsewhere, and works really well.
"Don't run Discourse on Cloudron" seems like a bad solution. Is this on a backlog somewhere to solve?