The breakout board is this: https://www.parallelminer.com/product/x ... -supplies/
Its got an ATX port, but it claims it can only sustain 180W through it.
I got a nice new power meter and have a full load on the CPU right now. It's reading 180W - 182W at the wall which means it should be ~10% less on the DC side. Correct? So I should be okay to run the motherboard through the ATX connector on the breakout board or should I use a standalone PSU to power it? Currently I am using a 500W EVGA PSU since I don't have any GPUs loaded on it yet.
The 24-pin ATX connector has got two +12 V pins, each rated for 8 A max AFAIU. That is, this 180 W specification of the ATX port of your breakout board looks to be simply about the same limit as per the standard.
Any additional +12 V power to a mainboard needs to be implemented with extra connectors. E.g. the 8-pin EPS12V connector has got four +12 V pins and is rated for 288 W total.
This image shows it clearly-
If you were going to attempt it maybe use a 6 pin PCIe cable with an adapter to 8 pin CPU instead of the one jumpered off the ATX like this - https://www.amazon.com/inch-6PIN-PCIE-P ... B01EEOW3CA
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That adapter should solve the issue. I'll just get two of them.