Personally - 7950x (16 cores - 32 threads) and most productivity reviews put it amongst the better ones. I think 3d is very much a mix of single threaded and multi threded operations so getting a high base/boost clock will certainly help.
Id probably want to go for more RAM, 64GB min really.
GPU wise, yeah 2 x 4090s sounds amazing... ive never used more that one so cant comment on how well they play together with rendering etc... I'd go for the higher end build versions such as ASUS Strix or MSI Suprim, all that rendering/ full load could take its toll on the cheaper end of parts.
I know ECC and Xeon are 'better' workstation grade parts but never really been overly sure on how much of that is really warrented for day-to-day productivity work.