1. 6000MT with more than two modules is not possible on AM5
2. Mixing / combining memory kits is not supported or recommended by the memory or board vendor. Memory kits are binned in the density in which they are sold and once combined with other kits the settings in which are programmed on SPD are no longer valid. Stick to single kits.
I carried out tests.
From frequency 4800MHz (AUTO) up to 5200MHz the boot times are 16-17 seconds, very good for 4 stiks of RAM.
From the frequency of 5400MHz up to 6000MHz (with Expo 1) I get times from 54 to 60 seconds.
I thought there was a more gradual variation from 5200 to 5400MHz.
The dilemma is whether to have 2 stiks (32GB tot.) @6000MHz with boot of13 sec. or 4 stiks (64Gb tot.) @5200MHz with 16 sec.
With 4 stiks of memory @6000MHz with shorter times I would not know what other Bios commands to act on.
I would try not to have such a preoccupation with POST times and use the amount of memory you require for the workload.
Enabled Memory Context Restore to bypass certain POST stress tests under specific conditions. This will help reduce overall POST times.
In other previous tests with 4 Stik, I had to set Context Restore >Auto, as it was on "Enabled" causing me instability problems.
It caused both OS and Bios crashes, where I could no longer act with the mouse, I had to act on the "Reset Cmos" and reset the setting.