I can tell you from my own experience that using an NVidia RTX 3090 has been a huge performance boost, even compared to my first Threadripper with 64 threads. There have been some issues, but nothing available today on a Mac can compare to the performance I see using the RTX cards when using KeyShot, mostly because KeyShot isn’t taking advantage of native Apple Silicon. If you’re buying something in the near term, I would highly suggest looking at a Windows machine since it’ll work well for you right now.
If you’re looking for comparisons, there’s a whole section of the forum dedicated to those benchmarks, and you can compare the numbers there. For me, a 3090 feels noticeably faster than my 64 core computer, and massively faster than my 32 core Ryzen.