If the Chromebook is your property, you can do whatever you want with it, and it’s unlikely that anyone will notice or care. I assume you’re in the US, since you appear to be worried about DMCA encryption-related provisions. Don’t be. Even if it were 100% guaranteed illegal with all necessary precedents, Google has better things to do with its time than track down individual jailbroken Chromebooks. It isn’t like you’re going to be selling them in quantity or using them to facilitate ransomeware attacks or something.
However, I’d invest in a used laptop instead, since it’s likely to have more internal storage even if it lacks the !!shiny!! factor. Chromebooks are meant to store as little as possible locally, and that isn’t how a normal Linux works. I suspect you’d start to get data claustrophobia pretty quickly.