So I recently posted here that I had gotten 3400 CL14 stable on my 2600X with one caveat: a "warm boot bug" where the system would hang during memory training if it was warm.

Yesterday I read that some people were having issues POSTing with Zen2 CPUs on ASRock boards and sometimes had to reset a couple of times or wait minutes until they got their machines to POST. This behaviour sounded suspiciously like my "warm boot bug" and the recent BIOS updates that include AGESA 1.0.0.3ABB appears to have mostly fixed this problem so I updated the BIOS even though "ASRock does NOT recommend updating this BIOS if Pinnacle, Raven or Summit Ridge CPU is being used on your system".

Update went fine, no new or changed options in the BIOS, but the erratic booting behaviour is gone and I managed to stabilize these settings:

https://i.imgur.com/7L7K85z.png

DRAM voltage: 1.4v, Vcore offset: -100mV, all other settings stock, GDM enabled.