Ph is good. That 600 isn't buildup per se, it's salts from the unflushed coco. That's what's locking you out and burning your tips.
Now, and this is just an option, you can give it a peroxide flush. This will do 2 things. Bust up the salts around the roots and release more oxygen into the coco. Most people will criticize the hell out of this, so I'm just throwing it out as an option, which will be quicker than the next option. I have done this and still do it when I flush. I like H202. Teaspoon to a gallon.
Option 2 is to keep feeding at your 200 with a fair amount of runoff twice a day. I would prefer some perlite in there so you could do 3 or 4 times a day, but it's ok. Keep the pots warm and a light breeze on the plants, just enough to make them wiggle. This will help it dry quicker and push the plants to suck up more water and nutes. You just want to make sure it's the right nutes at the right ph, twice a day.
Every time you water/feed coco it's always to runoff, always.