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?
Given this is for running a server for self-hosted, I would second what others have said here about using a VPS instead, provided you don’t mind self-hosting on a VPS rather than on your own gear at home.
You may be able to find a VPN provider that gives you a static IP and the ability to port forward. PureVPN offer this in limited locations and it’s what I use, but it’s jank since you can’t use their dedicated IP service with OpenVPN (their normal product does let you use it).
You may also be able to do something like run pfSense on a VPS, then run a site-to-site VPN to pfSense at home, using the pfSense in the cloud as your router. Then you can port forward from your pfSense VPS to your LAN. You’re getting very deep into the weeds with networking there though.