I like your change to make the AI stop building corvettes/frigates past a certain point. Those get pretty useless and laggy.

But I think the AI should still be building destroyers. The AI simply just doesn't have enough PD if they stop building destroyers, which makes missile or carrier builds very OP & cheesy.

Please allow the AI to keep building destroyers in their fleets, maybe it can make up a smaller proportion of their fleet if you prefer.