I think the limited storage is the cause of your reservations. If you could just buy a few large HDDs, you can pool them all into a RAID array and then store your library and downloads on said pool. The arrs can then create hardlinks which allow you to have "two" copies of a file while only using the storage space of a single one. One stays in Qbittorrent and the other gets renamed and moved to your library automatically. You retain the files until both copies are deleted.
As far as setup goes, Trash guides are popular as mentioned in other comments. I ran all this on Windows for over a decade but recently setup proxmox and host everything in docker containers now (along with a bunch of other non-media related stuff). It got to a point where everything started running like shit on Windows and I regret not setting things up on a Linux based OS long ago because my library has become quite large and I was essentially 'trapped' in an unfriendly OS with no easy way out (I essentially had to buy and build a whole new pool of drives and then move all my media over weeks to avoid losses), so I would seriously consider a Linux based setup if you're not already.
I have no advice on German media, but I would suggest trying to get into private trackers like you intend. Long-term seeding and freeleech files will take you a long way even if you have terrible upload speeds. I've built up dozens of TBs of upload credit with a 10Mbps upload speed just off bonus points and the little actual uploading I could do.