Adjusted watering pH to 6.4, light closer, more wind - the plant did a grow spurt in 3 days.
Watering when needed - around each 3-4 days, around 1-1.4l water. Light schedule changed to 20/4. Humidity - still getting hang of it, but median humidity is around 65% (still jumping from 50 to 72).
Overall the plant is healthy, stem is getting thicker.