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.