Linux

check out dell latitude 5285/5290 2-in-1. they are Surface Pro lookalikes with detachable keyboards, but with way service-friendlier interior - easy to open and SSD, comms, battery can be easily replaced, whereas RAM is soldered. the screens (12" 1920x1200 IPS mutlitouch) are gorgeous and the hardware isn't too shabby, kabylake (7xxxu) and kabylake-r (8xxxu), with standard UEFI BIOS so you can install Linux and have SecureBoot even. I can get them locally for $100-150, dependent on config and equipment (even less if they're without battery and keyboard).

edit: yeah, I misunderstood your idea, I thought you wanted a cheap linux tablet. what you actually want is a fantasy - an ultra-portable device with huge battery autonomy running linux and such a thing doesn't exist, for any kind of money.

namely, the mentioned dells are twice the heft of a normal android tablet and the battery autonomy is laughable; not only is it not an improvement over a normal laptop, it's likely to be worse, as that thing's essentially a laptop with extras, like touch, gyros, etc.

then comes the real hammer - touching the thing. Gnome and Plasma (and their derivatives) have touch support but if you're coming from an android or iOS tablet, that support is in its infancy. it's crude, inconsistent, flaky, and not very well propagated throughout the system. it's way better than it was a few years ago, but this is not something you'll want to hang your career or education on.

you can tweak the thing into something semi-usable, and for the price (around $100) that's a worthwhile endeavour and cute hobby project. it bears repeating, it is not daily driver material, and that includes way pricier solutions - saw a Ryzen 6-series the other day for like $700; everything I've written applies to it as well.