Using a proxy in qBittorrent means never having an open port for incoming bittorrent connections, you’re always firewalled / not connectable in that configuration.
I guess case is solved. This is exactly what I wanted to do, and it seems that it wont work.
What is interesting to me is that on rpi3 iptables -L shows tailnet IP with port 6881 listening. Also, wanted to note that pinging subnet ip of NAS from rpi3 works, but pinging tailnet IP ends in 100% packet loss.
Anyway, thanks for mentioning that. Guess I am stuck.