Welp, I don't think my cpu is a memory controller champ, wouldn't boot @ 6600 and OCCT memory test was barfing within 10 mins @ 6400. Though at that point I had no case cooling so the sticks were getting a bit toasty.
This was an easy tune though: 6200Mhz / CL28 / 2066 FCLK while nothing to write home about it's a nice balance, and since the sticks are dual rank I wasn't expecting magic anyway.
Couple of minor niggles, had to bump VDD to 1.52v for CL28, gear down mode is disabled in BIOS but not sure if it's either not sticking or Zen timings is reporting that incorrectly.
I read/heard the infinity fabric can get a bit prickly sometimes on AM5 with 1.3v SOC, so maybe I'll try again for 6400 but with the same 1.25v SOC I used here...
Edit: updated screenshot to show SPD pane instead