The trick is to ensure that sab/nzb/deluge etc are all in alignment with radarr/sonarr etc to get hardlinks support. So you need wherever your download clients are downloading to, to appear to be on the same drive when radarr etc look at it i.e. it's the docker mappings that count.
To make this as easy and foolproof as possible, ALL of my dockers have the same mappings:
- /user ---> /mnt/user
- /disks --> /mnt/disks
Just these - nothing else. So, that no matter what paths I use within dockers, they will always appear as being on the same 'drive' for inter-docker moves, and I get the maximum file transfer benefit.
I.e. for you within Sab, you would download to /user/downloads/sabnzb via Sab's settings. Radarr would look at /user/downloads/sabnzb (same /user mapping) and then move the files to /user/media/movies/whatever i.e. everything 'stays' in the 'user' drive.