TBH i don't know anything about airvpn, but a got a good deal on pia vpn for a few years and it has port forwarding which is good for torrenting. Their servers locations are also close to me so i get good latency and speed. And that's pretty much it.

I have PIA VPN + Transmission in docker and added my nas to the container via nfs. PIA is great because of port forwarding, however you cannot choose the port, so I made a little docker container which gets the port in PIA and sets it in transmission. It also stops the container if it disconnects, but that happens rarely. Also, all traffic from transmission goes through the PIA container. If you need more info you can dm me.

Pi 3b+ can you just need to enable it, look it up. Tried and tested.