Looking good mate. I agree its not a N def as the bottom fans are green- more likely a micro issue like fe as you say are the veins still dark green on affected leaves as that happens with iron def. If the whole leave is yellow then it could more likely be molybdenum.
You using any micro nutes? A foliar feed of those could do the trick and some epsom salts
I have Alga mix which i think is derived from seaweed,that should have all micro nutes needed,shouldnt it?