The GARDENA gardening expert
Whatever predator is feasting on your sunflower plants, Mr. F., is difficult to say without seeing the damage. I think that spraying is not necessarily worth it – even with an environmentally-friendly preparation such as CaterpillarFree or Spruzit Insecticide, both from Neudorff. First, the exact cause needs to be determined and then specific action taken against it. With damage done by snails, slime tracks should definitely be visible and only then would action be taken.
A more general help would be a cover for the young sunflowers with small protective hoods or “Lettuce Protectors” that you could use against any snail feeding on the lettuce. But even that is an elaborate undertaking.
The most likely solution that I see is an entirely pragmatic horticultural answer: Plant the sunflowers starting in April, in 10 cm plastic pots and after the frost, place them in the garden as soon as the seed leaf has developed the first few leaves. Then the sunflowers are reasonable large and robust. At the same time, they have rough, hairy leaves. They should no longer be appealing to the pests.