I use NetGuard on mine to prevent it from phoning home, and restrict internet connections to Firefox and a couple of other apps that I trust. I also mostly use mine with wifi off, since it's an ereader/notetaking device that doesn't need to constantly drain battery via wireless internet.

I am a little nervous about the trustworthiness of Onyx, but at the same time... I don't really trust my smart TV, or my old Kindle, or visiting friend's Windows laptops either. So I guess it's kinda moot.

It’s not moot. Choose devices with FLOSS like https://pine64.org/pinenote.

What is Onyx? I didn’t see that mentioned.

Onyx is the (parent?) company that produces Boox devices. I don't truly understand the hierarchy, the names seem to be used interchangably.