You have a Calcium deficiency for certain (Rusty necrotic spotting on leaves & red/purple stems on affected leaves).
Potentially a magnesium deficiency from the yellowing of the leaves starting mostly bottom up.
Since you're growing in Coco you should be making sure you're adding Calcium, Magnesium & Iron into your feeding regimen EVERY watering. I have learned over the last year the hard way that if in COCO you forget cal/mag you are screwed. This is why I switched to AdvancedNutrients Micro>Grow>Bloom because it already has plenty of Ca/Mg/Fe.
Botanicare sells a product called "CalMag+" aka Calcium, Magnesium + Iron. Ca/Mg/Fe.
Add 1 TSP/5ml of CalMag+ Every Watering up until 7 days before harvest and your problems will go away. Never add nutrients at all for the most part for the first week of transplant or growth. Just good water at around 5.5-6 PH ideally with coco. Personally I threw out CalMag and I literally only use Advanced Nutrients 3 part "ph perfect" Micro, Grow & Bloom and have had zero issues and don't need to be concerned with PH as long as I apply nutrient solution daily with waterings.
2 oz of Micro
2 oz of Grow
2 oz of Bloom
Stir & Water
Another thing is make sure your water temps are around 68 F ideally and your room temps are around 70-75 F.
Also if switching to A/N MGB you don't need to buy into their other stuff.. Just run the base nutrients, then if you want go buy their $40+ quart supplements and see if it makes a difference. Personally I tried Big Bud & B-52 supplementation and I still ended up with ~7 oz a plant in 5 gal containers using 2000w Mars Hydro LED COB lights.