Any Sifus can advice me some of my questions?

Currently I'm using 70mai Pro and I need 1 more dashcam for my rear view. It's sad that RC06 only compatible with A800.

Is it worth if I upgrade my 70mai Pro to 70mai A800 (with rear cam) or I straight away buy another dashcam maybe 70mai 1S or another Pro be my rear cam? Which mean Pro + 1S or Pro + Pro, much more cheaper than upgrade.

Question 2, if I install another dashcam instead of upgrading, can I use one of the dashcam connect to other USB port (my screen having extra USB port which I used to connect pendrive for my songs) since 70mai Pro only offer 2 USB port for the cigarette adapter, I used both port for my phone and also current dashcam.

Question 3, are those dashcam work with my screen USB? Im not expert in electrical field and I scare those electricity that come from my screen USB will burn my dashcam due to unstable or overpowered electricity. I also found that hardware kit are able to connect my current dashcam and enable the parking surveillance mode, but will it harm to fuse or the car or even the dashcam? Most important is it worth?