Honestly, not much to criticize on that rig. What I would change:
- CPU is a matter of money. 9900k might be a bit over the top. 9700k will do the same job for less money and even might overclock better.
- AiO cooler is a matter of choice. If you are going to overclock, by all means go for a good AiO. I still prefer a good classic cooler (like top of the line noctuas) for anything but the overclocking rig. Also, unless you really know what you're doing and are an enthusiast, for custom design waterloop cooling. It's not worth the fuss for anything but hardcore overclockers.
- As for RAM, I'd rather go for 2x16 kit rather than 4x8. Also, when dishing out for computer that much, be sure to pick better SKU. 3200 MHz is fine but pick stick with lower latencies (cl timings). Or go for a model with higher MHz rated model.
- PSU is a bit of an overkill. That's not a bad thing by any means, but 850 W would do the same job for less money.
- I would definitely look into a better case for that kind of rig.