Well it seems Ardupilot already has the capability to save the home position on first arm. It’s Home_Reset_Alt. Default is 0, meaning will reset home each and every arm. -1 turns the feature off so it won’t reset the home position after the first arm. It can also be set so it resets home when the altitude is from 1-127 (meters?). I’m not sure I understand how that would work, but setting this to -1 worked and didn’t reset home when I disarmed and rearmed in flight.