And since we will be able to customize all the vehicles starting positions then I will have it start at the terminal and it should return to it.
The customize starting position is an option of the "Customize *Parking* positions" so it's always related to a specific parking, even if you cheat and move them over the terminal as if the parking was very large, they will continue assuming to be "inside" a parking spot, and will drive to the airplane door without taking into account any AFCAD paths, because they don't expect needing AFCAD paths when "inside" a parking, regardless of large it is.
As I've said, the only vehicles that start/end at a terminal and follow AFCAD paths (and vehicles paths by preference), are those you CAN'T customize their starting positions: FollowMe, Fuel Truck, Catering Truck and Passengers Bus.
The one you can customize, operate only inside their parking, with the aforementioned exceptions of the arriving baggage train on Boarding, and the returning on Deboarding.