The only device connected to the nas will be the Mac mini to put files on and the tv for plex.
In this case, you have not much to benefit if you use connect both ethernet ports for aggregation.
I’d like to torrent to a separate hard drive on it if it’s possible I currently use transmission on the Mac.
Yes, as already @fredbert advised you can create a new pool with that drive and use a volume on that pool for torrenting destination.
For much newer packages you can install Deluge or Transmission from SynoCommunity.
For even better experience for torrenting with vpn and very frequent updates you may need to go for Docker containers. Since it seems that you still need a lot of help, I would advise to stay with SynoCommunity packages untill you have a little bit more experience to try Docker.