Update: #1
I spent more time with the memory post-review. successfully lowed it to CL32. It requires 1.4v
Update: #2
Running 6400+ on a lot of MB is still a challenge. I suggest updating to the newest BIOS for everyone.
While I did not have to change the supporting voltages for 6400 on the ASUS Z690 Hero, I keep seeing posts with ASUS Z690 Strix not booting it still. Try to manual input these voltages at 6000 if you are stuck and work your way up.
6400 (MB Stock) SA 1.25v / 1.385 VDD2 / 1.35 TX
6400 (Manual) SA 1.1 / 1.2 VDD2 / 1.15 TX
6600 (Manual) SA 1.2 / 1.35 VDD2 / 1.30 TX.
6800 (Unstable OC) SA 1.25 / 1.40 VDD2 / 1.40 TX
Note that for the Z690 Hero, 6800 would not boot without 1.4v on the DIMMs and manually adjusting the supporting voltages. However as you can see the stock voltage applied is higher than necessary for 6400.