I’m aware most ISPs do not allow for port 25 to be open for email use outside of business licenses, but at what level is that controlled? Can I get around that by owning my own router? Owning my own modem or ONT? Or is this just a thing they mystically control further up the pipeline that a relative layman such as myself can’t get around?
You’re already using an SMTP relay so why not host your Mail-in-a-box server at home? Been doing that for years. Also, check out Mailcow if you’re interested in running your server as a docker container.
I’ve been running my own mail server since ~2002. For many years I was using qmail, of all things, on a home ISP connection. I wrote a semi-popular guide on adding spamassassin support to qmail. I was a true believer!
When hosting email from consumer internet became untenable, I migrated to digitalocean and Mail-in-a-box. To be honest, it’s worth the $15 to have a 100% always-on device hosting the email. I host lots of other stuff at home and having email be a separate thing makes it much easier.