keep your data turned off. you load the tablet with books or whatevers, you don't need no comms, hence nothing can leak.

second, you should get the device based on LineageOS support, so check their devices page first. there's but a handful of those, I was lucky to find a Tab S6 Lite for cheap and it's hella supported. if you have money to burn then look into Pixel tablets and such.

finally, when you eventually flash them, the battery life isn't great. even just having them lying around not doing anything in standby requires you to connect it to power every other day or so. had an iPad like 10 years prior, that thing could be left for weeks and still be available when you need it. sadly, no such thing exists here.

p.s.: not what you asked for, but take a look at old 2-in-1 tablets in the vein of Dell Latitude 5285/5290/7290 etc. those are fully featured i5/i7 machines, tons of RAM, expandable storage, you can install Linux on them with all the benefits and drawbacks that brings; battery longevity also ain't a thing here, but at least you can tweak everything (limit frequencies, hibernate, etc.)