So in theory, you should be able to set that to the IP address that you have when you're connected to the VPN and curl curl -4 ip.me and to the port Proton assigned to you.
Edit timer ran out before I could fix this sentence. Sentence in proper English:
So in theory, you should be able to set that to the IP address that you have when you're connected to the VPN (you can use curl -4 ip.me for that) and to the port Proton assigned to you.
and here:
you should get incoming connections at some point.
I meant specifically inbound IPv4 connections since I assume that's all we're talking about. You have no problems with inbound connections on Tor, right?
I just found in jlopp's config generator under networking that there is -natpmp. I have no experience with NAT-PMP but sounds like it could be useful for your case.
So maybe a network configuration like this could work (replace IP address with the one from Proton)?
# [network]
# Accept incoming connections from peers.
listen=1
# Specify your own public IP address.
externalip=185.159.159.140
# Use NAT-PMP to map the listening port.
natpmp=1
-natpmp
. I have no experience with NAT-PMP but sounds like it could be useful for your case.