I have 2 zucchini plants growing, one has large healthy leaves, but it's young new leaves appear with "pebblely/curly" texture. The large leaves do have powdery mildew, which is being treated with Serenade treatment. This plant is showing signs of slower growth than it's larger more healthy looking companion zucc. No curly leaves on that one.
Zucchini Leaf Issues
Certified GKH Gardening Expert
I think you are treating correctly, but I would add dolomitic lime, and wettable sulfur. This will help kill off anything in the soil, and stabilize the pH. This is likely caused by the fluctuation of the pH from treatment.
Apply these, and you should see recovery soon.