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)