It sounds like you're on the right track, the fact that there is growth is a good indicator. I had a fungus gnat infestation on my fresnos and it the growth looked like your leaves on the first photo, so check and see if there are any little bastards crawling around. The "experts" say that peppers are more calcium dependent than many other fruits so try some CalMag Plus, it works great for me on RO water at about 6.5pH. Depending on how you water them, they seem to like more at a time than I thought when compared to other plants, it seems that watering them until you see at least 20% runoff has better results for me than just making the soil moist enough. A good soil that worked, at least for me, has very high drainage composed of 40-50% pumice and horticultural charcoal/superchar, and your soil looks nice from the first photos, but seems "depleted" and appears "faded". Last season I only used Superthrive on them with a good "calciumed up" water and got a bigger bush than a 70's porn star budy.