I think I figured out how to auto-start my Prius when the battery voltage gets low. Now to see if it will work!
I have a Victron BMW-712 installed to monitor my second 12v battery, the BMW-712 has a generator start feature that is customizable.

My though is to connect the BMW-712 generator start/stop to a DynaGen ES52 Auto Start Controller which will be wired into the Toyota Prius power button somehow. The thing is, I might have to leave a key fob in the car for this to function properly… That would mean wiring in a separate door lock/unlock relay and remote controlling the door locks with the key fob in the car.
Stay Tuned for more!

I might be able to use a findmall DSE702K-AS DSE702AS Auto Start Generator Controller which is cheaper than the DynaGen, but I need to do more research.