Bought a D-Link DIR-859 A3 AC1750 a few days ago. After some research, discovered that the components are almost identical to the LEDE-supported DIR-869:
Qualcomm QCA9563 SoC
Qualcomm QCA9880 802.11ac radio
Qualcomm QCA8337N GbE switch
16 MB flash, 64 MB RAM
No USB ports
Although physically they look very different, the only relevant difference I see so far is the DIR-859 has 8 LEDs while the DIR-869 (I don't have one) only has one or two (white/orange).
So I installed
dir-869-a1-squashfs-factory.bin (17.01.4 stable 10/18/2017) to the DIR-859 via emergency room. It works very well so far.
The power LED and 4 LAN LED lights work as expected, but the WPS button is mapped as the reset button, so pressing it resets the router. A recent Nov commit fixes the power LED mapping for the DIR-869, but moves the power LED functionality to the internet LED instead for the DIR-859.
I'll do a git pull request to once I figure out the correct gpio mapping of the buttons and LEDs for the DIR-859.