Im hoping to be able to move the factory starter relay to under the dash. That will require a 2 gauge wire that comes off the master disconnect and go directly to the main stud on the relay.
If you are moving the starter relay, there is no need for such large wire to it. If you are using that stud for a junction point for other loads, just run the main cable to the starter, and smaller gauge wires from the starter battery stud, and the starter solenoid stud, up to the starter relay. Size the battery feed to the relay to match whatever loads you are using. If you are using it ONLY for the starter, no 10 is large enough. No 12 "might" be. All you are doing is energizing the starter solenoid in the starter with that relay.