How do I migrate my Transmission torrents & settings from an old Macbook to a new one?
On your New Mac you do not need to install Transmission, in fact remove it.
Remove the HDD caddy with your 1000GB HGST hdd from your old macbook and install it in your new Macbook
On your new Mac customize the location of your Home folder so it points to your HGST 1000GB hdd
Take out and place your old Mac Vertex 4 SSD into an external 2.5 HDD enclosure, the Nexstar CX 2.5 is a great choice
Attach the HDD enclosure to the USB port of your new Mac
On the new mac type Migration Assistant in the spotlight search bar and the app will find your external drive
Select the external drive (orange coloured) and it will find transfer your applications, files, and everything!
You can include/omit 3 categories before migrating: files, apps, and network settings

When you open Transmission 3.0 (yes all othe Mac apps will migrated as well) all torrent colour tags/labels and torrents will be preserved. The file integrity check will be skipped, saving your hdd from checking everything all over again.
Notes:

Some apps on an older version of MacOS will not work once migrated to newer versions of MacOS. Download updates for them.
Pirated apps may not work depending on how they were cracked.
If you don't have an external hard drive enclosure, manually copy everything off your old Mac hard drive onto a usb key or external drive and it will work as well, but try to get the whole thing to fit into 1 drive. No partial transfers.