Awesome, now I just need to find an inexpensive shell of a Mac Pro 2019 - that seems to be pretty hard to find without a graphics card already in it. And it's the graphics card that rockets up the price. Trying to find just the chassis/motherboard/PSU with no modules (or at least no graphics modules) to build it on the cheap.
It's a interesting project and good luck finding all that you need, but you are mistaken with your RX-580 MPX module being the lion share of the cost. It's pretty inexpensive when you factor everything else.
Btw, buying an incomplete 2019 Mac Pro will probably cost you more than a refurbished one directly from Apple unless you are extremely lucky. The sum of the parts is always more expensive than the complete one. This never made any economical sense with Mac Pros after you factor all the costs involved - see the various threads in the past about this - at the end when the person factor all costs it's a lot more expensive overtime than a refurb/used one.
This will probably be even worse with the 2019 Mac Pro, since it probably sold a fraction of the previous models, with a lot less parts on the market and being new, less than 18 months since the first ones were delivered with another 18 months of AppleCare yet.