While, I'm not sure if you're in the US or elsewhere, here is my response from a US perspective. Another consideration is what carrier do you use, especially if you're in the US. A phone with a user-replaceable battery that is under $300 is going to be hard to find, at least new.

My recommendation for a new phone would be the CMF Phone 1. While the battery isn't user-replaceable in the "pop out" sense, it is designed with a more accessible battery for easier replacement than most modern smartphones. https://us.nothing.tech/products/cmf-phone-1

For used, this is also challenging. If you can find a Fairphone 3 or 4 in the US that works with your carrier, that would be perfect. The Fairphones have batteries that are removable by popping off the back cover. They are typically around $300 I think.