RAM speed, voltage and upgrade for Elitedesk 800 G6 Tower

[e5frog]

I'm planning to maximize RAM and its functionality, the computer with the option "4.1GHz Intel Core i5-10600K Processor" can take 128GB at 2933 MT/s. I mean to get 2x32GB as a first step and further along the way a second pair. 

Specifications about the various setups can be read here:
https://support.hp.com/vn-en/document/c06877008

As HP didn't have 2x32GB RAM to offer (at 2933 speed) I bought the cheapest memory config so I could buy RAM separately. The delivered 8GB was specified to 2666 MT/s - and it's correctly running at that speed according to diagnostics althought it's  3200 MT/s RAM DIMM:s from Micron. 

First thing I wonder, why is it not running at 2933 if 3200 is max? Clock steps/divisions?
What speed of RAM should I buy to actually get the maximum 2933 out of it?

HP:s virtual assistant directs RAM search to Crucial's homepage, you type in the type of computer and it lists a bunch of options:
https://www.crucial.com/compatible-upgrade-for/hp---compaq/hp-elitedesk-800-g6-tower#memory

All of the ones listed are 3200 and they are all 1.2V and listed as "Dual Ranked • x8 based • Unbuffered • NON-ECC".
I can find various 2x32GB packs with those specs at other places - but they all say 1.35V - should I be able to use them? 
Is RAM presented as 2933 the only option where I'd actually get that speed? 

I was looking at this type:
Kingston Hyperx Predator 64GB (2-Kit) DDR4 3200MHz CL16 DIMM 64GB 3,200MHz DDR4 SDRAM DIMM 288-pin 1.35V
...

But changed my mind after finding out the present RAM is 3200 but not running at 2933 (instead 2666)... also did not find a good explanation about the difference in voltage. Is it a max min, does the computer change voltage according to mounted RAM?

Maybe a 2666 with a low CL will be as fast as the available 2933 with a higher CL - don't know. I don't want to waste money "semi upgrading", might as well go for best specs if possible even if it's a matter of 1% in speed - it would somehow still bother me not getting the most out of it.

What will work, what's optimal?

Thanks for helping out. 

[e5frog]

Hmm. 

Intel presents the max RAM speed to 2666 MT/s for the i5-10600K - so HP is just a big fat liar?
https://ark.intel.com/content/www/us/en/ark/products/199311/intel-core-i5-10600k-processor-12m-cache...

... and I might just as well buy a 2666 RAM?