iMac Retina 5K 27-inch 2020 RAM Upgrade comatibility
As I am sure you are aware from the MacRumors thread you quoted:
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For maximum compatibility with 2020 iMac, you need four identical memory sticks.
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Get them from a dealer that guarantees full compatibly (including 2666 MHz) with the 2020 iMac.
Anything else, and it is a matter of experimenting.
You might notice that Apple Support page says "For optimal memory performance, DIMMs should be the same capacity, speed, and vendor" for the 2020 iMac, but not for older iMacs.
Studying published memory specs (e.g. CAS latency) is not sufficient for RAM in 2020 iMacs.