I agree about the 16 cores & 'diminishing returns' beyond (multiply core numbers x speed vs $$s to get a better feel for that).
In any case, FWIW, I do film work with Da Vinci Resolve Studio 16 & FCPX from time to time. Interesting to see the observation that 'graphics run at 75-100% and cpu runs at 100% '. I never, ever have had that experience & have monitored hardware usage regularly (Sensie, iStat Menus etc).
The most demanding tasks for me are always timeline caching & FX /NR, and rendering a timeline to H.264. Input media mostly multiple 4k files on a 1080p timeline; usually H.264 from consumer cameras, sometimes all-Intra from a GH5.
GPU runs at around 75-100% as mentioned, but the CPUs never get beyond around 5% or so (very unlike my dual Xeon Dell workstation where both RTX 2080Ti & the 2x12 cores go like the clappers). 192GB ram on the mac may have a little influence on that ... only 64GB on the Dell. All cache /proxies /optimised media is written to a dedicated RAID 0 Sonnet M.2 SDD PCie. Original media from a TB3 RAID.
In sum - *whatever* the tasks, the CPUs never really work all that hard, whereas the GPU can indeed top out. Three displays.
So I have no intention of upgrading the CPU (Xeon-W is a dead end anyways), but the GPU - absolutely. I ran a couple of Titans in the Dell at one point & clearly Resolve loves multiple GPUs. So the plan for now is to keep the Vega II doing what its doing and add a second 'compute only' GPU in the spare 16x MPX slot. Likely a big Navi should it come out later in the year, and of course if and when Apple supports it.
That's pretty much how I also thought about the mac config when I first spec'd it: 16 cores as best bang for buck; a single Vega vs. the duo (being fairly certain the overpriced latter would under-perform & become dated quickly) & deliberately configuring my slot usage to allow for a second, high performance GPU ... what a drag that cannot be NVidia, but maybe big Navi, who knows?