Robert
I've had the apples in my little suburban espalier garden in New Jersey for over 30 years. Our greatest challenge is pollination. We rarely see bees these days. Our local County government decided that we needed the entire are sprayed for mosquitos so they drove a truck up and down our streets spraying a very toxic pesticide to anything that flies.
I get relative small fruit set these days. Plenty of flowers. This year the Caville Blanc set no fruit. I purchased the trees from a espalier tree dealer out at the end of Long Island. He sells whips and some already trained trees. I believe he may have Cox's Orange.
https://www.henryleuthardtnurseries.com/ He apparently is responsible for the pollarded London planes at the UN in NYC.
Our summers are hot and wet which may be to liking of the Cox's Orange. I've adopted a strict organic approach to pest control.
Use Spinosad, Neem and BT for most of the issues that come along.
There is a web site run by Cornell University that lets you know the pest stage so spraying has a maximum effect. You plug in the nearest airport and it will give you degree days and stages of the pest and when to treat.
https://newa.cornell.edu/userSecond problem is that our local squirrels take fruit and runs off with it. I have solved that problem with a remay sack I put over the fruit when it gets big enough. There's even a twist tie on the top of the bag to allow easier fixation.
My real treasures are the pears. I have a number of espalier Passe Crassane, Beurre Grifard and Magness. The Magness was introduced in the 60's and does very well here.