Then you've got to consider what are you optimizing for. Is the power bill going to be offset by the cost of a Pi plus any extras you need, or a cheap second hand PC someone wants to clear out, or free if you can put an old serviceable PC you have already back into use. Is it heat? Noise? Space that it needs to hide away in? Airflow for the location?

That one's quite expensive, but there are others from Ugreen and Aoostar with less powerful CPUs.

I think they generally idle at 5-10W, like modern laptop parts do in general.

I've been eyeing off the Radxa ROCK 5 ITX with the Rockchip RK3588. There are two variants, one gives you 4x SATA, the other gives you 1x PCIe.

There’s also the Orion O6 if you need more IO/perf - https://radxa.com/products/orion/o6#techspec

I'm using a retired 4790k build for mine, idles at 60W and I really need to do something about that.