Hey Jean,
As I mentioned in my previous message, if you export to 1080p you will get a loss of quality, regardless of the kbps. That value ensure the best quality possible for a 1080p web file. But the amount of pixels in the image cannot change. You also need to keep in mind that a .png graphic is 300dpi, while video is 72dpi. Thus, you are effectively reducing the pixel density by more than 4 times.
So, unless you export to 4K, which is still going to look less detailed than your original .png, you are not going to get a better image that what you have now.