If you're mainly gonna read pdfs, an ereader is probably too small to use comfortably.

Look into eink note taking devices instead. They're basically similar to a kindle, but also usually have a wacom layer to use their pens to annotate pdfs. Also they're usually bigger (at least letter format) which makes them way better for that stuff. Most support epubs too.

I used a reMarkable for some time. It has a cool homebrew community, since the creators are pretty open and just give you the root password. They also have their own sync apps and subscribtion stuff if you prefer that.

At least Sony and Boox have some similar devices. Some come with Android which might be handy for you.