Using Syncthing with Proton VPN

Here it’s said relays have a rate limit:

“Relays will have rate limits of 50-100kb/s, so unless you are in Antarctica, thats not a better solution.”

And from my understanding opening a port for WAN is not secure. I am not using a VPN because of syncthing btw, I am using a VPN for privacy reasons in general with or without Syncthing (protonvpn also has a great adblocker and geofence bypasser) I’m wondering how to connect to Syncthing securely with protonvpn