The only thing you can't beat is the proprietary GPU with two gpus chip, potentially four gpus talking to each other, also 128G HBM2. I'm not sure how much benefits it will bring in terms of real world performance. But according to official Mac Pro info about GPU, depending on which software you are using, you get 2-5 times of performance improvement compared to iMac Pro. Cinema 4D and Davinci Resolve benefit the most, FCPX not that much.
If we use X299 try to match/beat, 10980XE OC + 3 Radeon VII (not sure X299 MB can fully support 4 GPU) should do a decent job. Of course, you can't beat 1.5T memory if you go with X299, you have to go C621 into TB memory territory.