The good thing about your tree is that it possesses abundant vigor, which any plant must in order to be a good candidate for pruning. It appears though to have not received appropriate pruning for espalier for several years. In addition, it has overwhelmed what appears to have been a marginal trellis in the first place. I would first propose that you replace the existing wood trellis with a post a wire trellis better suited to supporting a mature espalier tree like this.I would not undertake any pruning now, I would harvest your apples as usual and use colored tape or some other means to identify which major branches and twigs are producing which cultivars. Apples are harvested when they pass the taste test, that is, when plucked from the tree and bit into, they are ripe. The will likely ripen at different times. The fact that some are falling indicates some of them are already ripe. After the leaves fall, it will be far easier to see what you are dealing with. Build your new trellis to accommodate however many main horizontal cordons the original owner had. Just from looking at the whole-plant photo it looks like there are three main cordons, two going to the left and one to the right. I would expect four, but perhaps one cultivar failed. Remove any major vertical shoots other than the original main trunk. I see several of these. After this, it is a matter of removing the many watersprouts that each horizontal cordon has produced so that the fruiting laterals on each cordon are not totally shaded.

The extension publication in the link below, "Training and Pruning your home orchard", describes espalier in one part and provides further detail on the technique. This should get you started but please feel free to write with questions.

https://catalog.extension.oregonstate.edu/pnw400

Thank you for your prompt and informative response.

I assume the pruning is a winter project, before spring growth can begin.

Correct?

Mike