I'd really like to just run my own Postfix, Dovecot, SpamAssassin stack, but it seems totally discouraged these days just on the basis of email sender reputation.

It's not as bad as some make it out to be. Check out Stalwart, as it is much easier for a newcomer to mail hosting to manage.

Just do it. You're allowed to have more than one address. You can keep using Gmail for important things while you experiment with your own server.