There is something called "light castration" or "light sterilization". Look into it. Keep the aggressive duck in a dark environment longer than the sun is out and it can reduce his mating drive.
Duck behavior varies. There is never a one size fits all solution, duck personality will play a part as well. Sometimes you can have combos that are unexpected.
Keeping them seperate will protect them and is probably the best solution for now. When you change the flock dynamic, expect some more fighting initially until they get pecking order sorted out.