Ok let us cover all your bases. I just skipped back to your seedling feed strengths. I use ppm 500 scale. But your numbers seem low if my head math is correct. Normally is.Got problems in my tent. Perfect temps perfect rh perfect ph. But plants don't grow our die in coco. Thinking might be some pests I can't see. Going to replant what's left in new coco and put sand in bottom and on the top surface. Heard somewhere that kills them by the sharpness of the sand cuts them little POS. It's got to get better than the last month or I'm thinking of taking some time off as this is just to stressful. Banging my head against the wall long enough
Have you rinsed the coco super well? Especially if I came in a brick. I run multiple gallons through just one little 1 gallon pot.
Are you making sure to water daily to significant runoff?
Those are the only things I can think of really.
Beware of sand on the bottom as it can block the drain holes. I always layer 1 to 3 inches of perlite on top of my coco as it will nix the bugs no sand needed .