Japanese maples grow best when planted in well-drained, acidic soil that is high in organic matter. While they can be grown in poor soil, their growth rate is much slower and trees are more likely to experience stress. Ideally, they should be placed in a spot with dappled shade. Lack of nutrition and lack of shade may be contributing to your tree's lack of height. Although your tree looks really healthy and beautiful. Slow release fertilizers are the best choice because they break down gradually over the course of the growing season and supply a continuous source of nutrients as the plant needs them. These types of fertilizer will have a high percentage of “water insoluble nitrogen” listed on the label.  As for pruning, I have attached a link to an extension folder on how to prune trees and shrubs.   Remember, pruning stimulates plant growth, never prune more than a third of the growth in any given year.  https://extension.unh.edu/resource/basics-pruning-trees-and-shrubs-fact-sheet