1st Rover Build - Traxxas Slash 4X4

Greetings folks,

Working on my first Rover build using the following equipment:

Transmitter: Spektrum DX8
Receiver: FPV Racing Serial Receiver 3.3V autobind
Vehicle: Traxxas Slash 4X4 with 2cell 7.4 V 5000ma lipo battery
Autopilot: Pixhawk Mini with latest firmware V3.2.3
GCS: Mission Pilot 1.3.56

I have a castle BEC 10amp peak 4.8v - 9v our connected to the output rail with hopes to power and deliver signal to the onboard Traxxas ESC and Servo. The BEC is powered by another 2cell
7.4V 5000 ma lipo battery
I also have separate Video components, but I’ll post later for input. My initial concern is getting this up and running with at least 2 modes:
Mode 1- Manual with servo control for gimbal
Mode 2- Auto navigate with servo control for gimbal.

I’m having the most interesting time trying to figure this all out. Could and would love some help in figuring out where to start. I’ve been through many of the forums multiple times. Where to begin ? Transmitter, channel assign, mission planner ?