This is not going to sit right with you but you are actually doing the ducks a disservice by continually feeding them.
Like all animals, ducks will breed better if there is a ready supply of food available. You already realise that next year you will have hundreds visiting you. Unless you are prepared to feed them all, many will become emaciated as the environment can not sustain so many ducks.
You can have the best of both worlds by occasionally feeding them just a little 1/2 inch cube of bread per duck, this won't do much harm, or the occasional hand of pellets, but they will hang around for a while. Feed them from a yellow or bright coloured container and when you put that away (even cover it with a towel so they can't see it) they will soon learn that there will be no more food that day.