Hello James, have you got some info about your problem? I'm also on Amd Threadripper Platform, and honestly I can't reproduce the behaviour you described.....about it, could you be more specific? so I could try to reproduce something similar.....
on the other aspect I completely agree with you, from the render point of view, is simply a beast!...on other aspects, I have no such a complex model to put it in crisis, and in addition I think I'm not completely aware of which part of the program is capable of multithreading operation(from what I can say, are background drawing rebuild operation, layout refresh and publish, and clearly cinerender.....if there are some other please notify me).
Waiting for other POV...
Cheers
Francesco
AC 6.5-27 | Latest build | Win 10 Pro 64 | AMD TreadRipper 1950x 3.4 Ghz | 64 Gb RAM | AMD Radeon 5700 XT 8GB