Yellowing on lower leaves and some yellow/brownish spots on fan leaves in the middle. What could be the issue?

is that soil or coco? if soil why 10% runoff? you flood out microbes 'n shit, no? I would say you got a pH problem here. toxicity and a deficit of a mobile nutrient (thats why the older leaves turn yellow) You sure the pH is around 6.4? not like 5.4 or something? you should still add CalMag, even if tap water EC is 0.4E.C. Are you by any chance in the USA? because their tap water is usually why their plants fall apart, you gas off your water before watering? extra: you should remove the lower flower parts, this will just turn into popcorn buds and "steal" energy and resources from your tops buds, good luck! (:

The thin and bent down leaves dark green are a sign of nitrogen toxicity. Too much nitrogen and other minerals can't be absorbed correctly. Mulders Chart. You are correct in still giving grow it into flowering. Just maybe back off for now and only do the bloom for a few waters and see if it gets better. Just my thoughts. Curious what others may say. Best of luck! Happy Growing!