When my last cheap mp3 player died, I got one of these to use as my mp3/reader player.
https://www.amazon.com/Samsung-Galax...s%2C176&sr=8-5
It has a replaceble battery, which I replace every year. Like you, I put everything on a sd card so that I have plenty of storage for my ever growing pile of books. I won't get a phone that is less than 5 inches, it's just to hard to read on anything smaller. I would look for a cheap Samsung and call it a day.
Stay away from Blu products, I bought one of their phones and after a year it died!