It looks like Mozilla removed the Boox review, presumably because they didn’t update it. In episode 259 of the The Privacy, Security, & OSINT Show, I remember Michael Bazzell explained why he still prefers Boox. The main takeaway is that their privacy policy isn’t good but it’s kinda besides the point.
He didn’t create an account or provide any contact information and only connected it to his network which routes everything through a VPN server. He says while using NextDNS he didn’t notice any shady connections, it just looks like it was checking for updates. This was all years ago in 2022 and could’ve changed by now, but it’s worth mentioning.
I’m not sure if you can sync with Dropbox and Google Drive while retaining a whole ton of privacy, I’d spend some time thinking of how you could achieve what you’re looking for in a more private way. Perhaps Proton Drive?
As for general advice, I think the easiest option might be to avoid signing into anything (I think Boox eReaders are full Android tablets, so you can install apps without Google) and use a VPN which allows for ad/tracker blocking such as Mullvad. Ideally you’d do something similar to Michael Bazzell where everything connected to your network is forced to go through a VPN, that way devices can’t circumvent anything, but it might be too much to setup just for an eReader.
You can also look for alternative eReaders which might be a bit better for privacy, from my memory they might be more expensive or suffer in size/features: