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Well, it seems to be as simple as the PSU. Figured with it being new and I've never had any issues with Seasonic it was unlikely but I've done about 10 successful power cycles over the last couple days with no hangups. Thanks for the suggestions.

@Tushon said:
Bump voltage up a bit to allow memory to come up to higher speed?
but an otherwise functional memory kit should be able to get to speed with a little extra juice, right?

It's not just a DRAM issue. I'm a bit outdated on the matter (no time) but on release there were several issues:
1) BIOS wouldn't set correct voltages automatically (as per spec) - This was fixed for my mainboard later by a BIOS update
2) There were some memory dies that had higher chances of hitting their advertised speeds (praise "Samsung B-die" etc.) I believe the reason for that were also incorrect automatic BIOS settings. - But in general it's believed that 2nd gen Ryzen mainboards (b450/x470) are better for high speed RAM
3) Nobody knew what to do with SoC voltage (the whole subsystem of the CPU including memory controller)

Therefore the memory doesn't need more voltage because it probably works as it should. But there're definitely issues with getting the memory controller (tied via the new and shiny InfinityFabric) to play at high speed+low timings.

Here are the cranks in my order of importance

• Memory timings
• Correct memory voltage (VTDDR = Memory voltage/2, this was not automatically set correctly for me at launch)
• Correct SoC voltage
• Memory subtimings
• Luck

OP, if you have that option somewhere in your BIOS, you can let the system have more tries at booting memory at set clocks (I've found a limit option on those memory training attempts on my board). The fact that it boots sometimes is a good sign, there must be very little to change to finally achieve it.