There is a build guide (by KGP) for the older X299 boards, but I've not seen a guide yet for X299X. You're on the bleeding edge there!
If I I may contribute:
One of the two Asus X299 Sage motherboards seem to be more stable and reliable than most of the other X299 boards out there. I have WS Sage and it is light years better than the Asus X299 Prime boards I've had in the past. Others have the Sage/10G, which is very expensive, but has two 10G ethernet chips (to work on Mac, they have to be hacked, however).
I've heard of the X299X C621 Gigabyte board, but don't know of anyone who's tried to do a hackintosh with it........sounds very interesting, but until you have one, who knows?
I know of one guy who swears by his Gigabyte Gaming 7 X299 board......so there you go......
And, with my WS Sage, I simply added a 10G
Sonnet Syba card (about ~$100 dollars, which is natively support in macOS, and drivers exist for that card for Windows and Linux), and so, by going with the WS Sage, I saved myself $400 over the Sage/10G.....
There are posts on github and on the X299 threads which will give you a fair leg up and getting them up and running.
@kgp's last board was a Sage/10G on Mojave, so that's not too old, and his guide can still be useful.....