potassium K or late Phosphorus deficiency Possibly?

Looks more like a root zone / irragation issue. Waiting for a proper dryback is good, but are you fully saturating the medium? If you are choosing a volume to give and not giving what is needed to fully saturate, that's likely theproblem. i would not do a 'flush' but maybe get a little extra runoff for an irriation or 2. Feed as you normally would. you amened some peat moss, so how you fertilize is a supplementation of what remains in the soil... like riding a bike, you have to figure that out on your own. The plants don't look lush, so i doubt you are over-feeding. i would not reduce feed but if symptoms continue, change formula of feed for sure. again, i think this is more about the root zone. the damage is all around the veins to start. Check any marijuana leaf symptom chart and you'll see this is difficult to match up to anything... the location and progression don't match. the plant doesn't look overly lush and you say the pH is balanced (6.7 is fine for soil).