What’s the problem with just putting Linux on a Windows tablet? If you don’t trust a Surface to not spy on you with a new OS, the solution probably has to be two devices. There are made for Linux laptops that are light, but I don’t think any are super foldable or tablets.
Windows tablets are about as expensive as Apple tablets, and the Linux-for-Surface project doesn’t have some features working yet (e.g. Sleep mode on Surface Pro 4, which is the one it looks like I can get cheap used right now, and the Type Cover for that same tablet is wonky)
Ah, fair. For cost efficiency and everything working together without spying, then, your best bet is probably a Dell Developer Edition laptop. I think they go cheaper than any of the only-Linux manufacturers. You might be able to get an outdated iPad for your in-bed use, too, but I don’t know how much those get discounted.
Dell Developer Edition
Good Lord I’ve never seen these before. Those are fantastic.