For me, the X270 ist the sweet spot: it is small and portable and has a acceptable battery( in fact 2 of them). The display is usable and comes in full HD. 720p is a little bit too low for my tastes. You can upgrade the RAM and put an m.2 ssd inside(although only with 2x PCIe bandwidth). You can also charge it with USB-C. With an i5 it goes for around 200€ in Germany. US prices are usually lower.

Thanks for your work and don’t push yourself too far. I‘d prefer a good and carefully written app in a few months over an unstable und hasty written on in a few weeks.

There is the Sovol SV07 for about 300€-ish. It comes with Klipper and should check all your boxes. I bought the SV06 Plus two months ago it works OK. You have to tighten all the screws and actually read the manual, but that’s it. If you want a guaranteed problem free OOTB experience, you have to buy Prusa (top quality control and software) or Bambulab (not really hackable but runs well) for 600-800€.