While I think it would be too hard for most people to be completely free of proprietary software, atleast he is practicing what he preaches. It is a nice goal to someday get there, but I don't think its realistic at the moment.
Kind in mind, though, he is 72 and I don't think he even codes anymore. His computer use probably only consists of mostly Emac (for all text based work) and a web browser (which I read he has a very particular method that involves something similar to wget, lynx, and konqueror). His computer use is very light (I imagine) compared to many Linux users.
While I aspire to and appreciate what the FSF advocates, I don't see a realistic path for myself as a Linux gamer. The proprietary firmware limitations alone would keep you on 2015 hardware.
Source: https://kottke.org/15/05/how-richard-stallman-does-his-computing
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.