Pin oaks are acid-loving (low pH) tree species and amending the soil fertility with magnesium may very well be the cause of your problem; many magnesium based fertilizers increase soil pH (alkaline). With that said, foliar analysis, rather than soil analysis, is by far the most accurate means of diagnosing nutrient deficiencies in trees. Please retest the soil around your tree (take multiple soil samples and mix) to determine pH. Compaction to the soil, from construction of your new sunroom, could also be a factor in your pin oak’s general decline.
Concerning your question on pruning, please schedule an appointment with a certified arborist to assist you - since this question requires a “hands-on” kind of answer. No need to “paint” the pruning cuts on your pin oak this time of year; it will only interfere with your tree’s natural healing processes.
