I think pihole shouldnt slow down qbittorrent at all, but isnt pihole host using another DNS? Hopefully someone more experienced can jump in.
Anyway, use docker containers qbittorrent and gluetun. Setup gluetun for airvpn and bind qbit network to gluetun. Thats real killswitch. Alternatively you can set vpn in qbit settings, but double check is that 100% safe. Not all kill switch work like they should, and there is a risk of leaking home IP if VPN server craps out or maybe even on restart. There is also kill switch feature in airvpn client (eddie), but Im also not sure is it good as gluetun. Im using docker gluetun and qbit, just because lot of people recommend it