$3000+ price tag for a desktop. Wtf?
I'd rather buy the already well-established and recognized M3/M2 Max with maxed out specs.
edit: ie. sometimes you need a machine with 64+ cores running constantly at 3Ghz, that is very different from a MacBook
Apple actually does have a full-size tower type machine with slots:
https://www.apple.com/shop/buy-mac/mac-pro
I don't think it uses ECC DRAM, though previous Mac Pro models did. They also used to support up to 1.5TB of DRAM (though without the advantages of Apple's current unified memory architecture.)
As I understand it, the GPU issue is a software/driver issue rather than a hardware issue, so in principle one could create a macOS GPU driver that worked on the mac Pro, or port Linux (perhaps a future version of Asahi) to it and use a GPU that is supported with a Linux driver.
Look up what other workstations with similar specs cost, this is normal pricing.