[Quoth]

It's more nuanced than that.

Anyway, the problem ISN'T registering, which even on Amazon can be done almost anonymously. It's people that shouldn't know what you are reading, knowing.

So I don't any longer fill in books on Goodreads. An Amazon Kindle is hardly any issue, the Amazon app is dreadful.

Adobe have a terrible track record on privacy and security (Flash, Acrobat, sending book data in clear text) so I'd never register a device with Adobe, not with what I know now. I'd remove any Adobe DRM.

Anyone that has any issue to be concerned should use a Kobo, only transfer via USB and possibly block various destinations on their firewall if using the WiFi on the Kobo for Wikipedia or Google in a book. You can use a totally imaginary email. It's a pointless gadget to use on the Internet generally.
The Kindle isn't spyware like the Amazon app, but best to never connect it to WiFi and only buy books & transfer via USB, don't use Kindle Unlimited or Prime or KFX direct Whispernet download as that all triggers additional Amazon monitoring. The way to register Kindle without credit Card or real details is to create a working but fake email address only for that Kindle. By default Amazon assumes you are in USA, at least last time I did it. Use a real Amazon account, but login on USA market place and send a $1 gift to the new email address (it must actually work). Then on Kindle Registration screen use the new email and the Gift voucher code instead of Credit/Debit card. That's as anonymous as Amazon gets. Maybe your supermarket sells Amazon gift cards? But it has to be bought in USA (or whatever default Country Amazon is deciding).
Then never connect the Kindle to Internet again.

Both Kobo & Kindle can get FW updates via USB (except the crippled Kindle Kid's edition)

Use only Firefox or Waterfox on laptop/tablet/phone with a blocker like uBlockOrigin or uMatrix.

If they live in a "Tricky" country they need additional precautions.