There still are not many choices for real 4K dashcams, and for image quality, still only the Thinkware U1000 gets close to the A129 Pro, but you wouldn't use that as a rear camera! Action cameras are generally not suitable for use as dashcams, with the possible exception of the Gitup F1.
The Viofo A119 V3 is an excellent choice if you don't mind the size, it is not 4K, but it is twice the detail of a 1080 camera and has a decent bitrate so that you don't get that blocking. Does a bit better than the A129 Pro in the dark, which given that you don't have headlights on the back of your car, can be an advantage. It is also going to support HDR in the near future, which may be very useful for reading plates in the dark when they would otherwise be overexposed.
If the size is an issue then the Gitup F1 supercapacitor version is 4K and has a lens suitable for dashcam use, it is really an action camera though so the auto exposure is not as good for dashcam use as a Viofo dashcam.
There is always the option of another A129 Pro if size and money are not an issue, but I think for the rear I would go for the A119 V3 unless you are going to use it for movie making and want the better image quality of the Pro.
