For a new build I would go for Raptor Lake or Zen4. Not so much because of cost/performance, but because they already have support for PCIe Gen5, DDR5, etc. So as a platform, they will last longer.
Zen4 might have an edge as it will support several generations of AMD CPUs, so a few years from now, you can easily upgrade your CPU to a Zen5 or Zen6, without having to change everything.
5800X3D is definitely not a good price-to-performance CPU. To make proper use of it you need to either have the highest-end GPU (6800XT/3080 or better) or play very specific games like WoW, MSFS etc that are highly CPU dependant. If you pair a 5800X3D with something like say a 6600XT or 3060 you're just throwing your money away if you play typical AAA games.
What seems to be an early benchmark of the forthcoming Core i9-13900KS has leaked, and that's a processor that would launch early next year. So take this with a bit of skepticism in regards to validi...
I don't know about that, in the latest two most cpu demanding games (spiderman and cyberpunk), the 12100 does not show any signs of slowing down. Games are not as parallelizable as people tend to think.