Comment by baobun

Hard to know what ranges you have in mind with "decent performance" and "low-power".

I think your best bet might be a Ryzen U-series mini PC. Or perhaps an APU barebone. The ATX platform is not ideal from a power-efficiency perspective (whether inherently or from laziness or conspiracy from mobo and PSU makers, I do not know). If you want the flexibility or scale, you pay the price of course but first make sure it's what you want. I wouldn't look at discrete graphics unless you have specific needs (really high-end gaming, workstation, LLMs, etc) - the integrated graphics of last few years can both drive your 4k monitors and play recent games at 1080p smoothly, albeit perhaps not simultaneously ;)

Lenovo Tiny mq has some really impressive flavors (ECC support at the cost of CPU vendor-lock on PRO models) and there's the whole roster of Chinese competitors and up-and-comers if you're feeling adventerous. Believe me you can still get creative if you want to scratch the builder itch - thermals is generally what keeps these systems from really roaring (: