What is the word to describe a level of understanding that is in between in-depth and average?
I would use intermediate:
[Merriam-Webster]
1 : being or occurring at the middle place, stage, or degree or between extremes
In the example sentence, you might have a basic level of understanding and an advanced level of understanding. But in relation to the particular task:
It requires an intermediate level of understanding to work on X, Y, and Z.
If you're not an expert but, you can get the job done, we say: "You have a working knowledge of X,Y and Z. US