Thank you for contacting us regarding your question.  I really appreciate the multiple photos, and it does help narrow down the possible issues. There can sometimes be multiple issues, and the first one you can see based on your pictures, is that the tree appears to be chlorotic.  Also based on your pictures, you can see the brown spots being initially regular in shape and in-between the leaf veins.  It looks like you are getting large about of leaf loss, which can be a concern.
I am including some article links to give you more information:
Penn State - has some photos, and gives more information on leaf spot - https://extension.psu.edu/oak-diseases
Michigan State - good photos, and more specific information on chlorosis in pin oaks - https://www.canr.msu.edu/news/severe_chlorosis_in_pin_oaks
It appears to be an important tree to you, and you may want to contact a certified arborist to come to your home to evaluate the tree.  Certified arborists go through training and additional training on tree health, and would be a better choice than just a tree company.  You can find out if there are any in your area through the International Society of Arboriculture, https://www.treesaregood.org, and then contact the arborist directly.
Many times they will come out to evaluate and give you assessments at no cost, but when you contact them you will want to verify that with them.