It could be that the combined 8k texture is sufficient enough to capture all the information in your sculpt? Pixelpeeping the renders, it does look like there is more crisp definition in the higher frequency details (crackly bits in the scales around it's lips) At 8k for a udim of just the head, you're most likely needing…
Here are the displacement bakes out of ZBrush (just the head again for the UDIM one.) Original files were 32-bit exr format and red color, I've desaturated them to be easier on the eyes. Something else strange - on my duplicate model with only 1 tile, it showed like 90% of the displacement without any subdivisions turned…