Just some pointers from someone using coco...
Start your small plants in small pots when you can. I realize you're running autos and the idea is not to transplant them if it's not necessary, but a coco grown plant doesn't mind cramped quarters. In fact they do better this way and there's less of a chance of 'over watering'. 3' to 4' plants can be easily grown, with great success, in 1 gallons pots. Bigger is not always better.
With that being said, you can and should feed your coco grown plants every day, if not multiple times per day. This is the advantage of coco and it's the advantage you as the grower should exploit. Straight, 100% coco is fine (it's what I use), though some will add perlite; it's your choice, but for me it's unnecessary. (When I have seedlings, they get watered 1 time per day...when I find their pots getting lighter from feed to feed, I up their feed amount and start them on twice a day feeding...and it remains this way until they're harvested.)
Don't try and reinvent the wheel by using the latest and greatest nutrient offerings from Joe Bob's Garden Supply...'automatic this', 'super duper that', 'biggest buds ever', 'automatic pH', blah, blah, blah...there are very simple products on the market that yield awesome results without all the mumbo jumbo horse shit. KISS...Keep It Simple Stupid (I'm not calling you stupid btw)
Feeding to runoff, while ideal, is not necessary, nor is it a deal breaker. If you're a salty feeder, you're a salty feeder...your plant will reflect this, no matter how much runoff you're getting. If you're not over doing it nutrient wise, regardless of run off, your plant will be fine.
Keep your pH @ 5.8 and let it naturally swing to 6.1 or so...this ensures your plant is getting the full spectrum of nutrients available to the plant.
Ca and Mg are needed more in a coco grow than other types of grows, because of coco's ability to hold onto Ca and Mg. I add these two components separately, in other words I don't use CalMag, I use Epsom Salt for my Mg input and Calcium Nitrate for my Ca input. This keeps me from adding both elements when I may only need one of them, thus minimizing the chance of locking out nutrients because I added something I didn't need. For sure though, Ca and Mg are your friend in this type of grow.
Don't ever feed with plain water...always be sure your water has some % of feed in it.
Don't let your coco dry out....ever.
FYI...Your budding plants and the super slow growth, in the last couple pics might suggest a light leak or an ill working timer. Just food for thought.
Hope some of this helps. Good luck friend.