I run LineageOS on my Nexus 6, to get ongoing security updates. I also keep one other sacrificial phone running stock android with bootloader locked, so no more security updates, but I don't run anything on it but my banking app, since it's too insecure.
If you skip updates long enough, someone might find a security hole, and if you've skipped the update that fixes it, you'll be able to jailbreak it, install koreader, read epubs without conversion, use the filesystem for ebook organization.
Also, you'll avoid advertisements, which Amazon is now pushing to the homescreens even of kindles that were bought with the extra-cost no-ads option.
since podcasts are I think just RSS feeds of audio files (mp3 for those I've checked) the ads aren't in any way marked in the stream. The only thing I've found is adjusting the skip buttons in antennapod so that skip fwd does 10 seconds, and back does 5; that seems to let me avoid listening to most of the ad; tap fwd until it's back in material, then back once.
But I listen to a lot less podcasts; if I want hands- and eyes-free material I'm more likely to use TTS in my (text) RSS feed reader of choice, currently Feeder.
I don't do Windows, but I happily sync directories between my Android phones and my Linux PCs (especially a cloud server I lease) with rsync over ssh within Termux.
If you can set up an rsync server on Windows that should work. Besides actually implementing robust and efficient sync, rsync is also smart about platform differences.
For the specific case of Windows to Android, I've heard of people scripting up tools to shove all the contents of a directory over adb push.
Kindle isn't based on Android; it's bare Linux with heavy DRM and a very limited ebook reader app on it. Whether the MacOS kindle app would help, I don't know.
I run LineageOS on my Nexus 6, to get ongoing security updates. I also keep one other sacrificial phone running stock android with bootloader locked, so no more security updates, but I don't run anything on it but my banking app, since it's too insecure.