My Crepe Myrtle
Your crape myrtle is not looking good. It has nothing to do with pruning. It is really struggling. Is it planted by a road that is salted in the winter? Another issue with it is that it is planted in the ground too deeply. Scrape the bark of the bare branches with your fingernail if it is brown underneath the branches are dead. If so the branches need to be pruned out. But from the photos, it really looks like the tree needs to be replaced.