https://kindlemodding.org/kindle-models.html

Since “winter break”, it seems that most versions are supported.

I have a Kindle, and it’s relatively inexpensive (I assume the business model focuses on selling eBooks rather than the reading device itself). I never turn on the Wi-Fi or sign in; I use it offline by uploading books through USB with Calibre. I’ve read that it can be jailbroken, but it has met all my needs so far. Would this option work for you?

I also recommand Task.org app to sync tasks with CalDAV and I use raw Markdown files with WebDAV, both using the DAVx5 app for synchronization. For editing Markdown, I prefer minimalist apps that suit my needs. Right now, I find SimpleMarkdown (with disabled telemetry) to be the best choice.