

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.
Just mashing together shape primitives and Thingiverse parts in TinkerCAD is entirely underrated. It’s still primarily what I use unless I need particularly curvy corner.
Fusion360 and FreeCAD are the CAD versions of Photoshop and GIMP (if Photoshop had a restricted free tier). They’re both trying to be a legit piece of CAD software, so there’s a bit of a learning curve coming from TinkerCAD. I found it easier to “feel my way around” Fusion360’s UI. FreeCAD has a layer of, “How did Open Source devs decide to be different here?” on top of learning something new.