I, like you all, have been annoyed by the lack of enhanced graphics on the PS4 Pro. It boggles my mind how this could have even happened. The first example of a developer making the visuals for their game WORSE on the pro. And which developer does it? The one with huge resources, staff, and who spent 8 years developing the game? Wow.
Anyway, I have found that changing the output to 1080p sharpens the image. Of course, this is not a fix. Rockstar should fix it. But, set to 1080p on a 4k TV with upscaling: I think it looks pretty damn good. I also have the HDR on. I find the lighting to be better, but I set the HDR level to 250-300, despite having an LED TV. The recommended 100 level seems to dark for me. Also, I increase my TVs sharpness way up to 70-80. This really makes the details on arthur pop, I find.
Here are a couple snapshots taken with my Samsung S8+. The first one shows the 1080p output image, and the second shows the checkerboard 4k image. I prefer the first. I think the textures on the walls in particular are much clearer and sharper with the 1080p image.
1080p Image, with high sharpness on TV.
Checkerboard 4k, with high sharpness on the TV: