Classic coco problems man. That's potassium and magnesium deficiency showing up mid flower because you're doing feed-water-feed. In coco you gotta feed every single time or the cation balance gets all screwed up.

Feed every watering with runoff. pH 5.8-6.0 going in and let it drift to 5.7-6.1 over the week. Get 10-20% runoff every time to keep salts from building up. Don't flush with plain water in coco.

If salts are built up check your input vs runoff. If runoff is more than 150-200 ppm higher than what you're putting in, run some mild nutrient solution through there, like 500-600 ppm until the runoff comes down close to what you're feeding.

For GH 3-part in coco do 6ml micro, 9ml bloom, no grow per gallon. That's the Lucas formula for coco.

If you're on RO or soft water add 2-3ml cal-mag every feed. If your tap water is harder than 150 ppm just do 1-2ml cal-mag or use half teaspoon epsom salt instead.

Shoot for 1.5-1.8 EC going in, about 750-900 ppm on the 500 scale.

Hit them with half teaspoon epsom salt per gallon for the next couple waterings to stop that speckling then go back to your normal routine.

Don't let the coco dry out completely. Most plants in flower want daily feeds, maybe twice a day if you got small pots or it's warm. Water when the pot feels lighter and the top inch starts to dry.

Pull off the worst damaged leaves, they're not coming back anyway and it'll help airflow.

Skip the PK boosters for now. You're already bumping the bloom nutes and too much phosphorus can mess with your calcium, magnesium and potassium balance even more.