I'm using mailinabox, very similar to mailcow. Before that, did all the config myself.

Incoming spam is hardly a problem. Spammassassin, rspamd and those catch most. Greylisting the rest. Once a year I see an uptake in spam, spend a few minutes dilligently marking everything a spam/not spam which the server the uses to retrain itself a little.

Spamfilgering when selfhosting is hardly more work than on gmail, live, proton and such.

Your outgoing mail icw spamfiltering, however, is an entirely different, and tough problem.