Smaller plates make a difference to the portion size served and the satisfaction in getting through the portion.
If she asks for more, don't say no, but wait a few minutes for her appetite to settle. If she is still hungry, then she can have more. Over years of buffets at children's parties, the heavier children will tend to load their plate up with a piled up portion to begin with. Leaner children with big appetites will tend to take a more modest sample then return more times in tune with their appetite. It's not so much that there's a great difference in food consumed on that occasion, as that it's easier to stop in tune with your body's needs when your default is smaller.
Don't ban anything, make it more occasional.
The danger of swapping sugar to sweetners is that you're not actually changing the behaviour of desiring sweetness. Better to have the real thing less often than confusing the body's signals with diet/ reduced versions.
Put the right balance of foods on the plate. Vegetables are great, but don't go overboard as that's still training the appetite to go large when presented with other options.
OP, learning to cook a good range of simple, nutritious everyday meals without the lashings of very calorie dense ingredients will be beneficial. Cooking doesn't have to be complicated, and there are so many simple recepies and videos out there, it is easier to learn than ever. For every day cooking, a few techniques go a very long way, because they transfer across food types.