Get a dedicated reversing camera, dashcams are not really cut out for this job
First of all dashcams have very little screens, and for reversing you will want a 7" screen
Dashcams dont have guide and distance lines like reversing cameras have, and also the image are nor mirrored so on a led screen the curb on the left side of your car will be on the left side in the image on a screen.
And dedicated reversing cameras + a 7" LCD screen are pretty cheap.