I have i7 6850K and 5930K systems as well as i7 920, 930, 960, 980X, and just upgraded my i7 7800X to an i9 10920X. In terms of tangible gameplay experience we have not seen the chains move too much ssince 2012/13.Could you maybe add 6850K to the bunch? In summer 2017 before 8700K launch I bought it and Asrock x99 Taichi for 510 Euros total. It was at the time less costly than 8700K and equivalent quality motherboard. It also provides 40 PCI lanes. Most importantly, it provided an upgrade path (to 6900K/6950X or Xeons) as the second hand market matures and used CPUs that are compatible get cheaper. This is in contrast to 8700K which is the strongest CPU one could slot at that time in that platform. Still rocking 6850k with 4.3 GHz OC.
Furthermore, you can get a used 6850k and x99 board for 250 Euros to 350 Euros total. Maybe in the US even cheaper if not the same amount in USD. Of course with Ryzen 3600 it has become more irrelevant but even Ryzen does not provide all the lanes (B550 or X470 might mitigate that with PCI E 4.0) and is most likely slower for high refresh rate gaming.