That's an awful experience... Honestly, I have only a few times used that kit even though I am well aware that that's a super super common kit of RAM. Now, here's what I am thinking... Don't blame the motherboard, blame the ram. I have a strong feeling that it's Hynix not ASRock, Ryzen in my experience HATES Hynix. I have gotten a few Hynix kits to hit close to spec (2933mhz on a 3200mhz kit) but, only 5-6 times out of 15-20 Zen based builds I have done have I actually achieved spec on a hynix kit. I have a feeling you probably just lost the silicon lottery but, you could never really know until you try another motherboard.
Also, super cool build! +1
Well check this out, I upgraded to BIOS 1.20 for the ASRock B450M Pro4 last week, and today I said let me try to OC again. For some reason Ryzen Master was crashing when I tried to OC the APU lately so I ditched Ryzen Master and jumped into the BIOS set the APU to 1500MHZ, bumped the GFX voltage to 1.2, SOC voltage to 1.2, and bumped the DRAM voltage to 1.4. system ran stable, then when back into the bios and bumped the ram to 2666 and I've been running stable now for about 3 hours at 2666 playing some pubg and black squad. I tried 2933 and 3200 and not stable. So I think my issue is a motherboard that has limited memory support but got a little better with the latest bios release, and I'm still cursed with the Hynix ram that now won't run higher than 2666. I need some good 16GB ram with Samsung chips!