Selling off a D800 when a D850 is present already to fund a D500 appears as a good idea to me (despite D800 has got some advantages as a backup-camera, as it is also FX format).
Regarding the usefulness of the DX format I see a significant improvement of the D500 over the D300 (and probably the D90). It has downsides regarding IQ, noise starts to appear at lower ISO rates and color rendition does not fully compete with D850 or D4S, but when I do supertele photography(with say a 600 mm f/4) and need range, the D500 gives me a range advantage that sometimes is crucial (unless the times when I had the D700 and the D300 and sometimes it was better to take the D700 and crop afterwards). Often enough I get better results with the D500 (maybe at the price of a not as optimal but still sufficient IQ) and used it more often than the D4S even when i had both of them with me. (that was the main reason why I skipped the idea of aquiring the D5 and decided to wait for an even more advanced model). Now that the D6 is here, DX format is still not obsolete and I'd wish for a D510 upgrade in the future.
BTW.: A Z6 found its way in my bag and is a good addition to my equipment but can't replace the D500 or the D850 for me (that would not be different with any other Z camera that is available now)