It is not uncommon for new leaves to have those reddish colorations initially. They usually go away as the leaves go from a pale green to a deeper green as they mature.
I do suggest that you try to improve the light if possible. It looks like that window is narrow and there is a building outside the window that may be blocking some of the light.
Worm castings are best used outdoors. Your Fig is not lacking nutrients of any kind, so just use water without any supplements.
Moisture meters are notoriously inaccurate so I suggest you stop using it. Assuming you did not add any soil to the top of the rootball, then water your tree as soon as the surface of the soil feels dry and is a slightly lighter color. When you do water add enough so that a small amount runs through the drain holes. As long as the pot does not sit in excess water for more than a day, then that is not a problem.