See: https://protonvpn.com/support/port-forwarding
These systemd configs will watch for changes to /run/user/$UID/Proton/VPN/forwarded_port as described in the support article.
When a change is detected it will trigger the service to update the qbittorrent config via the api.
If your qBittorrent api requires a password you will need to modifiy the curl command.
This assume user level systemd.