A few related pointers here: how to run webservices from a domestic internet connection, how to set up a Pi to use minimum power (in this case for solar-powered Pi), and finally how to run Discourse (the software used for this forum) on a Pi 4.
The trick to reaching a home server is a tunnel. Cloudflare now offer a free tunnel, you just run their utility on your server and it connects to their edge network and accepts connections:
Edit: also
https://www.jeffgeerling.com/blogs/jeff-geerling/raspberry-pi-zero-conserve-energy