Pruning old apple trees

More tree pruning questions Gojiberry - are we having a slash and burn moment?

It took me about five years to get some old canker ridden apple trees into shape and still one refuses to fruit reliably. On reflection it may have been better to replant with new stock of preferred varieties but it's hard to resist the challenge

Hard pruning and heavy fertilising is almost certain to cause reduced fruit yield for a few years as the tree puts all it's energy into fighting back - and probably a lot of fruit lost to bitter pit (or simply the June drop) as the fruits that do form get neglected by the tree as it diverts energy into growing new branches.

Old trees that are badly cankered would probably be best removed.

My suggestion would be to plant some new (canker and scab resistant) apple trees this autumn-winter and get them establishing over the next few years, while doing only the minimum amount of pruning to keep the old trees going for a bit longer.
If you're lucky, you might be able to slowly get the old trees back to health with light pruning for the next several years. If not, the young trees will replace them after your efforts have failed.

If you remove more than one-tenth of the live wood, the tree may not give much good quality fruit for a couple of seasons.
If you remove more than a quarter of the live wood, the tree could suffer shock.
If you feed a tree heavily after pruning, it may not fruit well for a couple of seasons.

Some of my greatest mistakes have been because I was too sympathetic to a sickly plant. A sickly and diseased plant is too much effort and often dies anyway. Better to face reality and know when to give up.