If low power consumption is important to you, check out Intel's N100 options. Decent chip, low power. Just make sure you get your own (reliable brand) SSD in. 

I don't know what you mean with no small PC's, since I saw you mention mini-ITX, also a good option, will be overkill but easier to swap/upgrade parts. Can also check HP/Lenovo/Dell mini desktops.

To me I also didn't want to use a major container service like Umbrel, set everything up from source code myself instead, but that's optional of course, but you do have the choice with these options!