I don’t have as much experience as some people here with espalier, but I’ve been growing up 7 espalier apples on my back fence for 6 years now. Also, a lot of this stuff is not able to be completely right or wrong and people follow different philosophies about shaping and pruning principles for espalier.
My feeling on your picture is that I would cut the big branch cluster on the left as you have drawn in purple. Yes, the general idea is to cut with three buds left over on the stub, but I think if the side stub is more than 1/2 the diameter of the main branch, it could stand to be removed entirely. This is from my interpretation of a guideline @alan has espoused elsewhere and I think it makes sense. Otherwise your whole structural layout will eventually be compromised. In your case that cluster is almost as big as the main branch and not going to get any smaller. Especially if you cut it in dormancy, there is some chance your main branch will pop some latent buds into shoots which could eventually turn into spurs or new stub branches. Though there is also some chance that won’t happen and you will end up with a bare spot. I guess I’m more willing than some to sacrifice a few potential apples for the next season in return for keeping the long term shape on track.
I could also see cutting the part on your right purple line, but I could also see leaving it and cutting the shoots to three buds.
BTW - very nice stone wall!