As good as the D850 is (and it's very good) it's AF is not in the same league as the D6 or Z9 - it's good, but it's not as good as the two newer, far more expensive options.
So my view is to sell the D6 only if you need the cash it will command to pay for the Z9, or if you are done with dSLRs - otherwise keep it, it's better at high iso than either D850 or Z9 and in some rare situations the AF will outperform the Z9. You also have the same battery system across cameras which makes life easier (not relevant if you are running the pro batteries in the D850 too)
Sell the D850 in all cases, it doesn't do anything the Z9 can't do.
And I'd also sell the D500 because it doesn't do anything the Z9 can't do and there is not enough difference between 21mp (D500) and 19MP (DX crop of the Z9) to worry about it. Its dynamic range is also the worst of the bunch.
In the end, you'll end up with 2 Z9s and you can trade the D6 to buy the second Z9