Gamer wrotePhone shops do offer battery replacement to non-removable battery phones. and if you are a bit tech savvy you can do it your self.
So don't limit yourself just because you want to have the phone for a few years more.
Also by then that removable battery might be hard to find
Yeah--many, if not most, local phone shops can replace batteries in sealed phones----the issue you will run into is that a phone which is opened may not retain some of the same characteristics it had, such as water and dust resistance that it had prior to them working on it so keep that in mind.
And if you want a phone with a big screen and good price, I think you have to look elsewhere. There are a lot of decent chinese phones with big screens, big batteries and little else you can get from Aliexpress, Gearbest and others for well less than $300. The Xiaomi Mi Max 2 for example has a 6.44" screen, a 5300 milliamp battery with a qc 3.0 for about $300.