I feel really upset that I decided to prune my mature magnolia before seeking out more information. I cut off some dead branches, some suckers, and one thicker branch, ~ 2 inches in diameter that was mostly dead.
I don’t think I took off more than 5% off the branches, but I am hoping I didn’t damage the tree. I used new tools and covered the large cuts with spectracide pruning seal.
When will I know if I damaged the tree? Is there any remedy if I did?
You haven't damaged the tree. It is a good idea to remove dead and making the plant more aesthetically pleasing is always good for you. All is well. As far as remedy for pruning, once something's been cut, it can't be reattached.