X470 Taichi: mem frequency locked with Samsung OEM

You're wrong, it's there in plain sight in OC Tweaker. It just ends in a hard lock after selecting, saving and then restarting.

Also the timings can be changed and saved (still are set after reboot), but they never come into effect. Only the timings from the JEDEC profile are actually used.

There is an option for memory frequency in CBS.

It is to be found in Advanced/AMD CBS/UMC Common Options/DRAM Timing Configuration. Yet setting a frequency there doesn't stick. On saving and rebooting -> the setting reverts to AUTO (DDR4-2400 again), even when the cold reset method is used (like for CLDO_VDDP in NBIO Common Options). So this route is a dead end as well.

EDIT: I need you gizmic. You said that you have tampered with OEM sticks that don't come with XMP profiles on your X470 Taichi Ultimate and that they are overclockable. Can you without doubt confirm, that setting the DRAM multiplier and the timings works on your end with sticks like this?

Which ones did you test?

I ask because what I see on my board very much feels like a restriction by design and not like a bug.

EDIT2: I forgot to change OC Mode Change Switch from ASRock Setting to AMD CBS Setting. With the latter in place I get my usual hardlock upon changing memory frequency in AMD CBS.