Pro should be closer to XB1X, the consoles are only a year apart, no doubth RDR2 development begun on base consoles years ago but optimizing for Pro should be on par with XB1X. Something dont seem right something happned while developing on mid gen upgrades.
It might simply be a choice the developers made. Upscaling 1920x2160 has its benefits and perhaps with their rendering solutions looks better than upscaling 1800p. 1800p checkerboard might have also caused some frame rate dips, limited by gpu, while right now the pro is stable and seems to be limited rather by the cpu in some occasions.
What R* could have done (and what I kinda referred to in my original post as lack of enhancements) is offer a performance mode and one prioritizing picture quality. Having the vertical resolution native is great for wide screen open world games where you are always not far above the ground. Definitely a good choice to prioritize that.
What is surprising is that base XBox One is only 65% of the res of base ps4, or base ps4 having 1.56x the pixels of the base Xbox. If anything went 'wrong' during development it's there. Yet perhaps not that surprising as double the GPU rarely means double the pixel resolution. So the reverse is also true, you need to drop res a lot more to keep it running. If anything it suggests RDR2 development was done for the XBox One X and resolution turned down for the other consoles.