As a quick fix, I just used UD to mount a 1TB spinner that I wasn’t using for anything else. Then I pointed my /data paths for Deluge, Radarr and Sonarr there. It let me see if there would be an improvement without spending anything. If it solves your problem you can decide if you want to setup a raided cache to protect your torrents, etc.
Once the downloads are complete you should be able to copy them into the protected share without any issues.