I don't know much about sails and slugs nor nematodes. However, if you think the mulching is what creates the habitat for them to thrive, you could use different mulch that they don't like to live in or use some sort of ground cover instead like strawberry or clover. Another thought that comes to mind, assuming you can't have or don't want chickens or ducks, is to create habitat for snakes. If you created the habitat for the slugs, can you create habitat for the predators of the slugs? Another thing you can do is find more plants, like oregano, that the slugs don't like and grow those instead. More perennials, I am always planting more perennials.