Thank you for posting your good question (and a good example of this) in the forum!
I would start somewhere in the middle if I could hand have that be roughly the battery change spot. This would keep both takeoffs at the same elevation and you wouldn't be too far from the ends.
Another option would be to use the Linear Flight planning tool which will keep coming back past the middle point. If you did 4 or 5 passes you could change the battery at the end of one of them. You could also start up high and keep an eye on the aircraft all the way down. Maybe only walking down part way.
At any point during the flight you can hit the RTH button and it will drop a blue abandonment point which it will go back to. Or you can manually select any corner waypoint (manual restarting point) when the blue path verification line is showing which will draw a larger blue dot.