A very, very latent response, but I’ve finally figured this out!
It wasn’t the BIOS at all, it was the (lack of) amd-pstate driver being enabled in the kernel.
There is a chunk of information here: How To Use The New AMD P-State Driver With Linux 5.17 - Phoronix
The only difference is on kernels > 6.1 instead of the grub arguments they suggest, you just need a amd_pstate=passive on the kernel command line. If you don’t have that, it defaults to off and you’re stuck with the intel-compatible (i’m guessing) apci driver, which bottoms out at 2.2Ghz instead of a low of 500Mhz with amd-pstate.
You also need to enable CPPC in the BIOS (on ASUS, go to BIOS< F9 for search, type CPPC, and set enable on both options)
And finally, a graph:
