What’s wrong with Postfix? It was part of the first mail server setup guide I read, and its internal filter options to reduce spam and bot access early in the pipeline seemed a good argument.
Well, kinda off topic, regarding sendmail/pickup usage from Discourse.
Marking this as solved/answered. It makes sense that the Discourse container cannot access the host’s sendmail, no matter which server. So it needs to use SMTP submission, for which one can authenticate e.g. via Docker container IP range in Postfix, skipping passdb/userdb authentication on Dovecot.