I'm not an expert, but apples only grow on last year's new growth. So if you get carried away and trim away all the wild looking new shoots you will have no fruit for a year. Don't try to do the whole tree trimming in one or two seasons.
Where I live we have such alkaline limestone filled soil and water that iron shortages are a major problem. This shows up as yellow leaves with green veins in them. I spend more time putting iron supplements in the soil than anything else, as it doesn't last long, and too much will kill the tree. Your fertilizing will have to be in accordance with the soil and needs of your area.