Here is the Gigabyte Aorus Xtreme C621 motherboard. Build your own T2 chip free 2019 Mac Pro. You get to use any NVME SSD you'd like to. Put it in any E-ATX case you'd like to and have fun creating your own 2019 Mac Pro Killer.
The motherboard will only set you back over $2,000 USD. Ask your bank to raise up those CC spending limits. If you can get by with a paltry 192 GB of ram then this might be the best way to go. The Xeon W 3175X costs much less than the 28 core Xeon that Apple offers as their highest core count CPU. It sells for just over $3,400. Less than half of what Apple asks for the 28 core Xeon upgrade.
You can also save about $1,000 if a 16 Core Xeon is good enough for your needs and high 5.0 GHz overclocks aren't as important.
You should be able to easily keep the total cost well under 8 grand for this build.
The 192 GB ram kit should cost less than $1,200. So a Maxed out 28 core Hack Pro could be done for:
C621 Motherboard -- $2,065
Xeon CPU -- $3,492
192 GB Ram -- $1,149 (96GB kit goes for around $500)
Graphics $375 (single card) RX Vega 64
NVME 2TB -- $350
E-ATX case -- $250
1200W ATX PSU -- $300
Total Estimated Cost: $7,981 USD
Note that you don't have to use ECC memory with the Aorus Xtreme board. I've attached the QVL from Gigabyte so you can see all the ram options. Here is the server ram from Crucial and Samsung that most closely matches what Apple uses. Personally, I would simply go with the lowest cost 2933 MHz non-ECC ram that is supported by the Gigabyte board.
Start out with six 16GB DIMMs of ram for a total of 96GB of six channel ram. You can always double that later to 192 GB.
Gigabyte has tested most of the Kingston Hyper X ram (Crucial) and you can get exact model specs from the QVL.
If you definitely need ECC then go with one of the two below.
Prices taken from Amazon.com and Newegg.com on 12/17/19
A new Mac Pro with similar specs to the above costs more than double what the Gigabyte C621 based Hack Pro would cost. Even if you double up on the Vega 64 graphics card.