Ok, so are you on the latest BIOS version? Version 3.9? If not, it would be advisable to update. When updating, please be aware of the update notes on the BIOS updates page starting with version 2.3. You must update to version 2.3 BEFORE updating to any newer version.
Also, make sure you are on THIS latest version of the X399 AMD chipset drivers. You should update the chipset drivers to the latest version BEFORE updating to the latest BIOS version.
https://www.amd.com/en/support/chipsets/amd-socket-tr4/x399
Do you have additional intake fans in the front panel, or are the radiator fans the only front intakes?
Honestly, it might not be a bad idea to add a couple of extra intake fans and also a second rear exhaust fan. The reasoning behind it that I'm thinking is this. You have two 2080 ti's connected to the front radiator. That is dumping a lot of heat into the case so that the radiator for the CPU in the top has nothing but already hot air to work with.
Adding some additional front intake fans if your configuration can accommodate it, which I'm unclear on since I am not familiar with your front radiator configuration and whether it is blocking any or all of the additional fan mounting locations up front or not, would help to bring the overall internal temperature down somewhat so that the upper radiator has a lower mean temperature to work with. Because if, for example only, the internal air temperature is 60°C because the saturation is high from the twin 2080 ti loop, then the temperature on the CPU loop can NEVER be below 60°C. You can never achieve lower than ambient temperatures without phase change or refrigerated cooling.
So for our purposes, the internal case temp IS the ambient as far as the upper radiator is concerned. Whatever the air is in the case, that is the lowest the temperature of the CPU loop can ever be. The front loop can of course be as low as whatever the actual ambient temperature is outside the case, although in BOTH these instances, it is only theoretical that they could be that low. In reality, they would be at least five to ten degrees warmer than that because it would be rare or impossible to have 100% efficient cooling that allowed you to actually achieve ambient temp balance.
So bringing some additional cool outside air into the case that DOES NOT have to pass through the front radiator, will help to drop the internal case temperature especially if you also have some additional exhaust flow by way of a second rear exhaust since that case supports two of them. That will also help to reduce the internal case pressure so that the front fans don't have to struggle quite so hard against the probable high positive pressure conditions inside the case. Neutral or negative pressure conditions are favorable for cooling performance AND for fan bearing and motor longevity. The easier it is for your fans to operate, the longer they will last and usually the quieter they will run as well. They themselves will also run cooler that way, so that they are not contributing as much heat to the internal case temperature as well.
Just some thoughts. I don't think there is any magic bullet unless you simply have a pump that isn't working correctly on your CPU loop. Any idea what your maximum CPU temperature is? You state your idle temp but no mention of load temps and since those are far more important, it would be helpful to know.