Only put the RAM you need in the box, use peripherals with working ASPM, attach them to the northbridge PCI ports instead of the CPU's root ports, use wireless instead of wired networking, and don't attach a display.
Chiplet based Ryzen CPUs inherently have higher idle power draw. Monolithic chips like 5600G have lower idle power draw. The motherboard, power supply, and internal peripherals all need to be carefully selected to get a really low idle figure.