There are small SATA backplanes that allow you to fit 3 HDDs into two 5.25" slots (or 4 HDDs in 3 slots). You can find used ones for cheap (mine was 30€), and with some cheap tower case you could get something NAS-like with hot-swap drive bays for way cheaper
They shouldn’t be able to see anything except encrypted wireguard traffic. Are you sure all torrent-related traffic is going through the VPN (e.g. requests to indexers) and that traffic stops flowing when the VPN connection drops?
Use a VPN? If you’re using containers, you can bind qbittorrent to gluetun to prevent leaks
Have you verified that? I would start by checking (with wireshark) for any non-wireguard traffic coming from your machine