NuPhy’s got some interesting options as well. https://nuphy.com/collections/keyboards/products/halo75-v2-qmk-via-wireless-custom-mechanical-keyboard
The low profile space is a little tricky. It leans into column staggered ergonomic boards really quickly. Kailh’s Choc switch is as low as you can go, but those folks get really custom really quick. They’re not big on function rows or arrow clusters, so the next step after Keychron or NuPhy would be something along the lines of the Afternoon Breeze. https://www.afternoonlabs.com/breeze/
I can’t help much on the power draw side of this question, but one thing to look out for with a UPS is some sort of communication option. (Usually NUT over ethernet, but there are some USB options too.) Most modern UPS brands will have a plugin you can install on your Raspberry Pi and Mini PC that allows your UPS to signal, “Hey, I’ve got 3% of battery life, you actually need to gracefully shut down now.” It’s mostly useful for NAS applications with spinning drives, but it could help save your Pi’s SD card potentially.
It’s a pretty standard feature these days, but the cheapest of the cheap will omit it.