Just an (unsolicited) opinion - MVC pattern if one of the few mainstays, and it's like that for a reason. It might (or might not) fit your use case, but it is hardly complex. Also any other dev looking at the code wil probably know it.
Apart from that, I would skip framework if I were you. Web is littered with abandoned PHP frameworks which were all viable at some time. If it already works, just clean it up, refactor as needed and identify + resolve any performance bottlenecks.
I appreciate the unsolicited opinion regardless, but yeah MVC is just too complicated for what this website does. I may end up just going with some kinda routing package instead of a framework, not sure yet.
In that case I'm not sure I'd use a route package either. If it is very simple, you can create separate files for each page and include them based on the path or query string. Few (or 0) dependencies can also be an advantage, as it will be easier to update and maintain
Apart from that, I would skip framework if I were you. Web is littered with abandoned PHP frameworks which were all viable at some time. If it already works, just clean it up, refactor as needed and identify + resolve any performance bottlenecks.