Our tree looked like it was dying. I was relieved to read about a fungus which it probably has which looks really bad but won't kill it. I know that I have to carefully clean up the fallen leaves so it won't be infected again next year. I had planned on cutting some dead branches this year and trimming off some of the verticle suckers. The tree is quite old and these jobs were neglected. It bloomed beautifully this spring. I'm afraid to do the pruning now while the tree is stressed and wonder if it will help or hinder the tree to do perform this work.
Dead and damaged wood should be removed as soon as possible as those provide a wound for disease entry. Routine pruning should be done in late winter or spring after blooming. Try to remove branches that are crowding the center, reducing light penetration and air circulation. Annual or biannual pruning of crabapple is normal maintenance. More info on crabapple pruning is in this article. https://www.gardeningknowhow.com/ornamental/trees/crabapple/crabapple-pruning-info.htm