When the D850 was launched, a lot of people scurried to sell off their 810 cameras. I availed myself of the price drop to buy brand a new D810 from my dealer's stock which with VAT deduction cost me less than the second-hand offerings. It was put straight into my studio for focus stacking and high-magnification work and worked flawlessly for these tasks. I now augment it with a Z7, likewise purchased cheap. All the remote control stuff and A/C supplies are interchangeable.
The D810 is much better than the predecessor D800 for focusing manually. It also has a superior Live View capability again seen against the D800.
Enjoy your D810.