Born in Southeast (Portland), I’m now in SW Virginia…  In Oregon I’d inherited a homestead full of that age apple trees ...and cut my teeth on caring for - and replacing them..  I don’t know that we Home Orchardists qualify as ‘advising Arborists,’ though we likely do it all the time for neighbors, family & friends...  

If your tree’s been damaged, or is being attacked by pests, or has major areas of rot & decay, it sounds like a job for ‘the professionals.’  And again, I’m not sure any of us qualify as such.  

Personally, I’ve done some serious pruning, and been paid.  My favorite subjects were old apple trees, as they’re very forgiving, and familiar.  But, remove too many shade producing ‘trees within a tree’ upright neglected one-time ‘water sprouts,’ or 2 inch thick mats of moss, and long shaded newly exposed scaffold limbs can be so severely sunburned you may lose them to bark decay and rot…  Leave ‘limb stubs’ with major pruning and you’re setting the tree up for major decay as well..  

Otherwise, removing unnecessary growth during the dormant season, when you can see it and not damage too much foliage removing it, and carefully removing thick moss, eliminating hiding and nesting areas for pests - is fairly easy and often obvious.  If you’re agile enough to safely get into the tree with a modern (sharp) ‘hand pruning’ saw (no chainsaw) ..and take your time, perhaps removing large neglected ‘upright growth’ and heavy moss over a 3 year period, you can do it yourself.  

As far as sprays or serious repair work, such as cementing holes, cabling, or inarch (repair) grafting, that’s doable too, with some reading and research..  Otherwise, some on-site advice or professional care may be the way to go.  

..gosh, after arriving in SW VA a month ago, I immediately tore into my neighbor’s two mature apple trees ..a couple days later with hours of ‘donated work,’ they look marvelous.  For some, caring for trees is a money making venture, for which few progress beyond the absolute basics, especially regarding the ‘fine pruning’ and care of established fruit trees … for most around here - it’s an addiction! - The compulsive desire to tune-up a tree to full productive capabilities and decades more of life.  

...so how’s that for a long-winded non-answer