Drtombanswered grow question 6 years ago I see your running coco coir and perlite, also says your running just two nutrients. Micro and bloom. If this is correct then you are running my exact same setup, although mine is flood and drain, your just hand feeding with slightly less nutrients.
You should do a couple things here, you should be watering with a higher strength dose of the two your running. Micro should be ran at 8ml/gal and bloom at 16ml/gal. Your much too weak at this point in the game. Are you running tap water? If so try to find a city/municipality water reading, it may tell you the PPM of your water. I run around 200ppm from the tap, but I'd try to be below 250ppm otherwise you'll want to look for another source such as RO water.
Also id be looking at getting some calmag, if you get GH. For that you can run it at 4ml/gal and you can even go up to 6ml/gal.
You can do this all with out a EC pen,ive ran it multiple times with excellent results, no burn and no deficiencies. Flush 2 weeks, before harvest with straight water.
Also Change your Ph, run it around 5.8 - 6.0.
As for the heat stress maybe move the lights up 6 inch and see if that helps.