Just to update things a bit, here's a thread that covers the ASUS ProArt 5K 27" monitor, amongst others, with links to some good reviews.
As for the new 5K Viewsonic display that's been mentioned, looks like
AliExpress has a page on it (and can ship to the United States?). Look over it closely; under Specifications it gives a contrast ratio of 1,000:1 and brightness 300 cd/m (which I think is the same as nits), but click View More and drop down to keep reading, and you'll see contrast ratio given as 2,000:1 and 'high' brightness of 400 nits. Says it has a Mac Color Mode and is Apple friendly. Their website lists it now at $1,400 but 30% off so $980.
The ASUS ProArt 5K 27" display is around $800. I wonder whether the Viewsonic has any competitive advantages to justify its higher price?