A full evaluation of the game's state seems like it would be very, very difficult, as there are many, many variables, and subtle nuances like team matchups would be hard to evaluate. This is why I suggest perhaps a more perturbative approach to figuring out an evaluation function. We start with a simpler evaluation function that takes into account only a few variables. The AI will likely make mistakes with such a function, but those mistakes can then tell us what variables need to be taken into account.
For example, suppose we start with the most basic evaluation function possible, the sum of the AI's team's remaining HP minus the sum of the opponent's team's remaining HP. We then notice the AI never takes time to set up SR. We then know that SR itself needs some extra weight, as the AI is not perceiving its long-term value.
To be honest, I'm actually kind of curious how well an AI would play using just that basic evaluation function mentioned.