There isn’t a one-size-fits-all recipe in most horticultural activities and bringing old trees into production is an especially good example. Variety and relative vigor are part of the list of variables.
The “decently structured” tree appears adequately vigorous to remove a lot more than a fourth of the “wood” -whatever that is- branches are one thing, canopy spread is another, taking out whole scaffolds is also different than thinning a lot of small wood from an existing scaffold. I have often removed up to 3/4ths of the smaller wood on an excessively dense, neglected tree and have never suffered severe consequences from removing too much- at least since getting a feel for leaving enough budded wood to get adequate sap pulled through the vascular system to avoid scorching bark. If there aren’t enough leaf buds to accomplish this, I believe the strong sun of mid-spring or so is when sap gets so hot it can burn living tissue in the tree. They used to paint trees white to reduce the risk of this happening.
That is a question not answered by the literature as far as I know- not something that’s been researched. However, painting trees white is still standard practice when commercial orchards change over varieties by removing virtually the entire canopy and doing cleft grafts off of butchered scaffolds.
By now, I’ve probably renovated about a thousand huge old apple trees, some neglected and some butchered- some butchered then neglected. I prefer merely neglected. Last week I was working in an orchard I’ve managed for over 20 years with century old trees- the largest have an incredible 60’ spread- literally an orchard in a single tree capable of producing hundreds of pounds of fruit.
They fell within the butchered category when I arrived- almost all small wood was being removed every year and some of the trees were dying. Very few apples were forming because there just wasn’t enough bearing wood.
I need to write a new article about strategies, but you can look up Ecological Fruit Production in The North and the chapter about renovating old apple trees. It was all I had to work with when I started my business over 25 years ago. Because my area used to produce the apples for the “Big Apple” on land now desired for building mansions I was able to build my business around rebuilding huge old apple trees.