When did you notice dead branches? has this been a slow decline or rather recent? Is there an oozing sap from the openings or is the darkness in the picture of darkening moss? It is difficult to distinguish. The crack is concerning. Can you take a picture of the entire tree for me please? Has it been sitting in water because of so much rain lately? 

Thank you for uploading the pictures. They help in diagnosing the issue. The pictures showing the cracks and black are a canker, where a pathogen has affected the stem, branch and trunk. This will affect the water conducting tissues and is serious. With the amount of canker and damage shown, I do not have much hope for the longevity of this tree. Here is a detailed explanation: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers.aspx

I would seek out an arborist in your area for another opinion. You can find a certified arborist here: https://www.treesaregood.org/findanarborist/arboristsearch   he should be able to tell you if the tree can be saved or needs to be taken down.