[Tobs40]

Author here!

Yes, if Gurobi and my code run as intended and I did not mess up any thinking while simplifying my chess model, then what I did is proof that the maximum number of legal moves available in a chess position reachable by a sequence of legal moves from the starting position is 218 (upper and lower bound). Gurobi proved the entire search space as "at most as good" using bounds, basically.