You dynamically request "a port" from the vpn gateway and it returns your port number.
As long as your nat-pmp-client keeps refreshing the port, it should stay the same. The timeout is rather low (60s afaik) so it probably wouldn't survive restarts.
There's a docker image that automates this for qbittorrent, but it shouldn't be overly complicated to adapt the script to other clients, if they can be configured via an API.
Because collecting data that is not strictly necessary is almost always a bad move. IP addresses might be relatively harmless, but might link you to other activities.
You personally might be okay with reddit knowing your IP addresses, but some people might get into trouble.
Take the insane anti-abortion laws in some US states. If an IP address from those states accesses pro-choice subreddits, that might be enough for law enforcement to start harassing someone.