At least in arch, the package qbittorrent-nox now contains the ability to connect to i2p. For people starting out, using i2p you wouldn’t need to use a VPN to download your favorite “linux ISOs”; just use i2p and have a fully automated Jellyfin server!
I recommend using i2pd as the i2p router
qBittorrent I have not tried personally but I would think that if you have i2p set up on qbit already then enabling the setting “automatically add these trackers to new downloads” and add in a few open i2p trackers. Postman requires an account so unless the exact same torrent has already been posted there you won’t be able to bridge.
For Prowlarr, it’s a little more complicated. You can add Postman just like any other indexer but you then need to configure your proxy settings in Prowlarr for it to be reachable. If you are running your i2p router on the same machine you can just enter your local address (127.0.0.1) with port 4444 and it’ll connect. If your router is on a remote machine the easiest solution is to then use ssh port forwarding (autossh is handy here). Ssh forward the remote 4444 port to local and then use the same address and port. The final step is setting your ignored addresses, I have a bit of a list but the idea is to filter out all non .i2p addresses so an example would look like: *.com, *.net, *.info, …
EDIT: I should also add, if you are sticking with I2PD and are more concerned with just downloading and not incorporating the *arr suite there is a standalone Snark download that’s floating around somewhere that can plug into I2PD. I haven’t used the standalone personally but I do know that Snark is by far the most optimized client exclusively for i2p torrents. Snark is also baked into the standard i2p install by default.