I think quite a few people buy XDR displays without a Mac Pro. The Macs that can drive the XDR are listed on Apple's website. It's only a few from 2018 up. It looks like the Blackmagic eGPU Pro would be the better choice for the XDR. I was looking over the whitepaper for AMD Polaris (the regular Blackmagic eGPU) and it cannot display HDR content higher than 4K resolution. The Vega whitepaper says it can display HDR content at 5K. It doesn't say anything about 6K though. Of course the regular Blackmagic eGPU should have the same capabilities as the Mac Pro's 580X GPU.
Yeah, sorry, I thinking of the eGPU Pro. It had something to do with the controller chip they used (as opposed to the controller chip in other eGPUs), but I'm not sure about the non-Pro model. I think they both have that TB chip, but maybe the 580 can't do it.
While several Macs can run it, I think only the new 16" MBP and the Mac Pro can drive it at 6k... (and then TB3 Macs using the eGPU Pro, I think).