How safe is it to write confidential info in my ereader?

Hi there! So I got a new e-ink tablet because I read and write a lot but also travel very frequently so can’t carry it all with me. Some of what I write is confidential and need to make sure it can’t be hacked. My idea is to keep it offline and write on a microsd card, then only use it online when the card is out. But I imagine it still could a) save some data on the device, either for usability (making it faster like cache) or by design (to make money with our data I guess?) b) connect to available networks on its own even if I manually set it off (yeah I’m a bit paranoid)

Are these concerns legit? Is there a way I could be 100% sure the info is only stored and remains in the sd card? Or maybe the only way around it is to break the wifi chip and sacrifice connectivity? (and still, my paranoia says it could have another chip somewhere.. i know ?)

Thanks for reading :)