I’ve got a kindle keyboard 3 and want to know if there’s anything better out now. My main requirement is being able to load in my own content and have a nice cover/case
I don’t care about apps or web browsers, but it would be nice to have something that is easy to sync or upload data to.
I personally don't think e-readers have improved drastically. I also have a 3rd gen Kindle keyboard and recently went through updating all the unlocks and screensavers I set up more than a decade ago.
While I was going through that it got me started thinking about them again. The only thing I think would be a big improvement is a light and more storage. I would also like to read manga on it. For those reasons I'm currently considering buying a Barnes and Noble Nook Glowlight 4 (or Plus) since it runs Android and you can install custom apps (like Tachiyomi for manga reading).
If your current Kindle is still doing the job and you don't find it lacking, I don't think you're missing out by not upgrading.