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.
Try the kobo brand of ereaders. You have access to an online library and can download through Wi-Fi, but can also load your own books in many many file formats.
I bought the waterproof version to read at the beach, while paddle boarding, in the bathtub.
Kobo is also great because you don't even need an account. That takes away some of the convenience features OP was asking for like syncing, but I love the fact that I don't need to hand over any personal information to Kobo in order to read my books; nobody knows what's on my bookshelf but me.
You can circumvent this by connecting to a router that has no access to internet. It will connect to the router, fail to connect to the internet, and then you can tell it to skip the initial setup and enable sideload mode.
While I really dislike Amazon I can email ebooks including pdfs and epubs direct to my kindle, I don't even need to go get it and plug it in and I can do this from my phone if I want.
If I have something really funky I can use calibre to convert and then email, or if it's really big I can always plug it in and transfer directly.
There are a lot of reasons to not like Amazon products but the kindle is actually pretty good considering who makes it.