I grow in coco. I totally agree that your coco looks dry as hell. I believe the ph of the coco spikes when it goes dry. I think 3 days is Not often enough for the feeds. Just feed them everyday and they will thrive. I feed my plants everyday until '25% ish of pot size' run off.
If you feel you have salts building up in the soil, or don't want to worry about that, just soak them with cal mag only 1 in 5 feeds. Wash all them salts out.
Another thing I found. I took shit advice from somebody at my local hydro shop about not running extraction whilst vegging. The leaves went light coloured with rust spots on, which is annoying as I grow 7 plants in 7 3ft x3ft areas. So I've spent the past month greening them back up again. Get some fresh air to those ladies, or seal the room and get some co2. Order some co2 bags, those rotting mushroom bags. lol
The plants are looking much better now I have up'd the feed to ec 1.4. I'm using canna coco a & b which has plenty of calmag already in, so providing you feed them enough canna a & b, they get enough calcium and magnesium.
I also use a root stimulator at the start. That with a fairly weak a & b feed sure gets those roots going.
I did do ph 5.6-5.8, but found ph 6 to be better. I believe my plants have greened up faster at ph6 than they were at 5.5-5.8. Now though, 6.2 is my cut off point.
I use straight tap water. I don't worry about all that r/o. Sometimes they even get fed chilled water from out the tap 5 minutes earlier. I don't wait for the chlorine and all that bollocks to evaporate. That's just a bit heavy for me.
I use 25 litre pots per 3ft tent, water once daily from go. Again heavy root stimulant feed at start.
Even autoflowers that are 8 weeks old from seed have fully rooted out 25 litre pots.