If qBittorrent/qb-nox is bound to your VPN interface, then 1) your VPN needs to support port forwarding, and 2) forwarding a port on your router is pointless and unnecessary. Your only way around it is to switch VPN or don’t use VPN and then port forward.
More people should use BiglyBT and its Swarm Merging feature. You get the ability to seed or download chunks from peers across separate torrent files.
It’s a shame because if more people used it, the BiglyBT devs might add hash-based merging (with v2 torrents) instead of just size-based. Hybrid/v2 merging is still possible, but file size is less reliable and caters to files only larger than 50MB.
Some kinda auto v1/v2/hybrid private<->public torrent maker plugin for BiglyBT would be… bigly.