Is 10W idle the lowest one can reasonably get a small home server? As a comparison, that's about as much as my fridge draws on average. Also, a mechanism to maximize the time spent at idle would be useful. Basically all the optimisations done for phones.
The Mac Mini M4 can draw as little as 3W during video playback via HDMI, but all your energy savings will be offset by the price of storage.
There are plenty of old laptops that would be a great replacement, mine draws 8W at idle with 2x2TB SSDs: https://www.kassner.com.br/en/2023/05/16/reusing-old-hardwar...
I’m interested in the next generation: arm64 machines with 2xM.2 slots could do wonders as an idler.