This is a simple little script that creates iptable rules and ssh'es into your phone to add your machine as a default gateway.
Note: This script just assumes your PinePhone's IP is 10.15.19.82 and the system you're running the script from has the same username as the one on your PinePhone, which might not be the case for you...
GitHub: https://github.com/CCF100/PinePhone-Reverse-Tethering
My Gitea server (upstream): https://ccf100.duckdns.org/gitea/CCF_100/PinePhone-Reverse-Tethering