you dont have distance or guide lines on rear dashcams, and the footage are not mirrored to suit so you will have to adjust to that too.
+ Dashcams while also wide angle the lens are aimed more strait out the back where as a reversing camera are aimed down a bit more to not have a blind spot behind the car.
Reversing cameras are also often mounted very low on cars, or on vans up high but aimed down.
Also. Depending on what type of car you have can get you issues, a sedan for instance, even with the rear camera high on the rear window, the boot will still create a blind spot behind the car where a child could be sitting.
It is 2 different beasts, and you are better off taking them on at their own turf one at the time.
I think a few of the mirror style dashcams do have the stuff needed on their rear cameras, but these are mostly frowned upon and something that see very little mention on this forum.