This is my current situation and it was surprisingly easy. However I only did it because it was more cost and time effective to buy two licenses and re-use hardware rather than buy new hardware at the time. I am currently in the process of condensing everything.
You can setup a SMB or NFS share on one computer and mount the share on the Unraid computer. Expose the shared path in docker and configure it in the *arr service. I have everything downloaded locally and the final file is stored on the NAS box. Everything is connected via a LAN. You'll probably get warning if a file tries to process while the Unraid box is off but it should not break anything and should resume when it can transfer again.
Not using a cache drive is not recommended, this post or others can explain better than I could.