Some tips here:

• get a platinum rated power supply, if you can afford it go for a titanium. The efficiency in the power supply is half of the efficiency of the rig
• reduce the number of the modules to the minimum
• get a platinum rated power supply ;)
• get big passive coolers, you want to idle the fans
• reduce the number of usb and connectors to the minimum. Their converters are not the most efficient. Try not to connect enything on them.
• NO mechanical parts (including fans or water coolers)
• set schedulers to conservative or power efficient. You don’t want to spike the power just because a task is 2ms longer than expected.
• pick a power efficient CPU/gpu (I think we can discard this one based in your choices)
• use the latest amd adaptative undervoltage technology to ensure to reduce the wattage of the cores
• try to reduce to the bareminimum the number of background tasks /services running.

And that’s all. Sometimes there is a component of trial and error because sometimes the curve performance / power is not entirely linear and you don’t want to hit exponential-non-linear zone.

Good luck and if you can post you build with numbers and some lessons learnt would be great

Good luck