I would say you can't get much faster than that, other than a small bit faster in single-threaded performance from an i7-7700k/8700K, but that would be at the sacrifice of the 18 cores you get with the 7890XE. I can't say for sure how it will run 4K 120Hz, but if you were going to try it 2x GTX 1080 Ti SLI and an i7/i9 would be the way to go. It will largely depend on the particular game's optimization.
They really went all out on that build. I don't have any better recommendations; but I'm also not real big on that expensive of hardware (out of my budget).
*I just looked at the monitors and those aren't 4K. They are 1440p, but don't worry that will provide a much better gaming experience (are you gaming?) than any 4K monitor.