I have done the exact same job. I filmed a 20km stretch of new motorway in New Zealand. You sound like you have your head around it. Don't worry about your home point. Monitor your battery life closely. At around 30% flick to manual mode (non GPS) and land. I filmed from the front passenger seat of a car with VLOS and my camera operator was in the back seat. There were no signal loss issues at all from being inside the car. Turn off Smart RTH and set Failsafe Hover. Even if the craft try's to return to home (which will be a few km's behind you) just flick to manual mode and take control and land safely.
Thanks for the instructions Craig! I finished 2 stretches this morning with great success, one for 3kms and another for 4kms, and all went well, good weather and very little wind. Like you, I stayed in front passenger seat, had VLOS at all times, flew at about 50 to 60 meters high (well above crossing power lines) and had someone scouting at back seat of car, driver going an average of 30-40 km/hour and never a glitch of signal, turned Smart RTH and set Failsafe to Hover too. Was able to land at about 47% batt level. Tomorrow morning will complete the other two stretches.
Did you fly 20 kms in a single stretch? I don't think my battery would last for such a distance. How did you manage that?
Rgds,
J.A.Sarmento