It looks like 2400Mhz is the best option if you don't want the RAM to slow down.  As far as I can tell, the reduced speed if all DIMMS are populated with 2133Mhz RAM is unspecified.  2400Mhz should run at 2400Mhz. You already have the manual, so I'm attaching the spec sheet.
I have a T7810 here with 2 x E5-2667 V4 and 32GB (4 x 8GB) PC4-2133. I would like to upgrade it to 64GB of RAM.   The manual states "If all DIMMs are 2133, the CPU ordered may run the memory at a slower speed."   If I fill the remaining 4 slots with 2133 modules what speed will they run at?   Would this system support 2400 MHz modules and if so what speed would they run at?
T7810 RAM Upgrade