I don’t necessarily think the 6.1 pH nor the LED lights were the cause of your problems. 6.5 is the pH to aim for in soil, but normally 6.0-7.0 is not a lot to worry about in a good soil. LEDs can be just as intense as HIDs for plants and the light being too close can be a problem even if it doesn’t feel too hot like it would with a HID.
All plants need to go through a “hardening off” when transitioned to stronger lights. This means slowly increasing the intensity of the light to get them used to it. It is the same when bringing plants from weak indoor lights to the outdoors intense sun.
The fact that you transplanted them is likely what slowed the growth, they need to establish their new root zone before you’d notice a lot of new growth above ground.
The leaves in the picture look a little “bloated”, as if from over watering, this can be a big problem for a lot of people. Be sure to let the soil nearly totally dry between watering or feeding. Then when you do water, soak to saturation once, and again let nearly dry totally before watering again. Determine this by the feeling of the plant and the containers weight, don’t water again until by weight it feels almost totally dried out.
Just some more info and things to think about.
~MacG