And this is how you can do it wirelessly from a Mac (iTunes being long gone):
https://support.apple.com/guide/mac-...0.15/mac/10.15
For some reason the device doesn't immediately show up in Finder sidebar, or I haven't figured out how to get it to. But it shows up eventually.
You can also use this to send .ePub to a number of iOS reading apps (e.g. Apple Books, Kobo, Bookari, Marvin, etc.).
(unfortunately AirDrop won't work because Kindle app doesn't have a Files folder to put it in)
I don't really require Sync most of the time, so I'm probably going to use this in favor of the personal documents service going forward. I like a more consistent reading experience (including those books from Amazon that were never updated for Enhanced Typography).
It would be nice to figure out how to do this with my Fire too. There's something called AirDroid, but of course it's not in Amazon Appstore so it would be another thing to get somewhere else. I also have a crappy Samsung tablet, which I'll probably try this out on first.