EC1.3 seems a bit low to me. The giveaway is that your smaller plants are greener and healthier than your bigger plants. That suggests a nutrient deficiency as opposed to a lockout.
I don't agree that N and K levels are too high. Potassium levels are a little high for veg, but not Nitrogen. An NPK ratio of 2.6-0.6-3.2 suggests those nutrients are more suited to flowering than growth. You'd be better off using a high Nitrogen CalMag supplement rather than Epsom salts, as you already have plenty of Sulfur, and the plants would benefit from the extra N.
pH6-6.3 is fine for coco, and in fact helps with Ca availability.
You can water coco as often as you like and - as long as you have good drainage - you won't have any issues. Unless you are recycling your coco, and it has started to decompose, the coir is coarse enough to keep large amounts of air in the root zone. If you are recycling, then as coco degrades, the granules get smaller, the medium denser, and there is less room for air.
But that only happens after you recycle a couple of times and leave old root mass in the coco. Also, fresh coco should not decompose so quickly as to affect cation exchange. Once you have pre-buffered coco with sufficient calcium and magnesium, it won't bind as much, and so these elements will be available with each watering. Magnesium is less mobile that other elements, and as others have mentioned, the lower temperature of LEDs compared to HIDs can affect transpiration and transport within the plant.
Coco benefits from more waterings - I feed mine five times a day on an auto timer - as the more you water coco, the more oxygen you introduce to the root zone, as it is pulled into the coco through the action of soaking capillary action and drainage out the other end. Drainage is most important, as it flushes the coco with each watering.
Likewise, if you water often, you must also use a reasonably high concentration of nutrient (upper end of the suggested scale), as you are constantly flushing, and so there is no build-up of nutrient. Conversely, if you do not flush often, there will be a build-up of waste nutrient that can lock out other elements.
By mixing perlite and expanded clay into your coco, you are effectively reducing your pot size by filling it with inert materials. So your plants will root-bind faster and may also show signs of nutrient deficiency - that is something else to consider (though your pots seem to be a good size for those plants).
My suggestion at this stage would be to bump your EC to around 1.6, replace the Epsom Salts with a higher-N Cal-Mag supplement (N Ca Mg 4-6-2, or something like that), and ensure you get a little bit of runoff with each watering and DO NOT let you pots sit in the runoff only to soak it up as the pots dry out - a sure way to increase waste nutrient build-up in your pots.
Other than that, they are not too bad. My opinion you just need to feed them a little more. In fact, if you pH test the run-off, and your pH is quite high, then that is a generally a sign the plants are using up most of the nutrient you have been giving them and may require more. I have been growing in coco for a long time.
