The mites are clearly a factor - at least some of that "deficiency" is necrotic spotting of areas where mites have been feeding. But there are likely other things going on, as your plant will be channeling energy into its immune system to fight the mites, which could also be causing part of the deficiency. Your situation is a little bit different to an otherwise healthy plant.
The one advantage you have is drain to waste - it should flush out excess nutrient as it waters, preventing a build-up of unused potassium salts that could cause a calcium nutrient lockout.
How often does your system water? What is your pH? You may want to increase the watering cycle if you can, to help with the flush and availability of fresh nutrient. A higher pH of around 5.8 - as well as letting your pH "drift" in the nutrient tank (naturally rising over a few days from, say, pH 5.8 to 6.3) - will make Ca more available.
Cold temperatures and growing under LED can cause Ca deficiencies - I've just been through this myself. LED light does not produce IR heat, but it does provide lots of light energy for growth. That extra light creates extra nutrient demands, but the lack of infrared heat means your plants do not transpire as much. This means elements that rely on transpiration for plant uptake - notably calcium and magnesium - can become deficient, even when you think there is enough Ca-Mg in your nutrient solution. This can be exacerbated in cold weather, or when humidity is high - as in your case.
High humidity slows transpiration, which can also lead to a Ca deficiency.
I got the quadrella recently: growing under LED, in coco, in cold weather with high humidity, I started to notice a Ca deficiency where previously I had not had any issues. The fix was a foliar spray of 20ml/litre of Cal-Mag, as well as a higher dose of Cal-Mag in my nutrient solution.
Try the foliar spray. It has two advantages.
Firstly, mites hate it - you can knock many of the mites off your plants by misting them, and then spray eco oil, neem or potassium soap around the base of your plants, pots and floor to kill the mites that fall off. Regular misting will help keep mites at bay. This is more effective than defoliating, IMO, because you are not removing leaves which are an important source of stored energy for the plant.
Secondly, foliar sprays act faster than root applications. Ca is a mostly immobile element. It relies mostly on transpiration (evaporation at the leaf surface) to get into the plant, and once it is used to build cells it becomes "locked" into the plant. The rust spots you are seeing in the upper leaves is calcium being broken down in the leaf in an effort by the plant to move it to new growth, where it is vital for new cell development. This is why calcium is mostly immobile or "semi-mobile" - it cannot be moved efficiently around the plant, and only small amounts can be relocated from nearby areas.
Ca deficiency will affect root function, so foliar spraying will provide Ca where it is needed and can be absorbed readily by the plant. My preference would be for a Cal-Mag supplemental spray including Mg and Fe, as usually where there is a lack of Ca, there will be a lack of the other two.
The only caveat is if you are already feeding lots of epsom salts or another source of magnesium that may be locking out your calcium. It is mostly unlikely but still possible. You generally want a ratio of 2:1 - 3:1 of Ca:Mg, and a ratio of 2:1 K:Ca. If you have a lot of salt (NaCl) in your water, this will lock out Ca. If you are using RO water instead of tap water - which naturally has calcium carbonate and magnesium carbonate in it (depending on your water source) - then you may see a Ca deficiency, as many bottled nutrients are designed for tap water and take into account the calcium carbonate already present.
It is important to know that the damaged leaves will not recover, and that the new growth will take some time to show improvement - especially late in flowering. Hit them with the foliar spray three times in a week and see how you go. But also bear in mind this could also add to humidity.
Finally, the foliar spray may lead to an increase in powdery mildew, but it won't if you have a good fan blowing on the leaves to dry them after each application. You can also adjust the pH of your foliar up to pH8 or so to combat the mold, as higher pH can prevent its growth. Epsom salts can also help, as it naturally raises pH and also has sulfur in it that prevents mold spores from germinating.
