I think a VPS and moving to NetBird self hosted would be the simplest solution for you. $5 per month gives you a range of options and you can go even lower with things like yearly subscriptions. That way you get around the subdomain issue, you get a proper tunnel and can proxy whatever traffic you want into your home.

As for control scheme for your home automation you’ll need to come up with something that fits you but I strongly advise against letting users into Home Assistant. You could build a simple web interface that interacts via API with HA, through Node-Red is super simple if it seems daunting to build the API.

If a RPi 4 is what you’ve got and that’s it then I guess you’re kinda stuck for the time being. Home Assistant is often quite lightweight if you’re not doing something crazy so it runs well on even a RPi 3, same with NAS software for home use, it too works fine on a 3. If SBC is your style my recommendation is to setup an alert on whatever second hand sites operate in your area and pick up a cheap one to allow you to separate things and make the setup simpler.