>>> досконально is a synonym of подробно and nothing more

Well, I would disagree here. You know that all of those synonyms are there for a purporse. The words are indeed similar, even almost the same, but they still have their differences. Examine this:

знать досконально - For me it means that you know just about everything on the subject, that you didn't even miss the tiniest, the subtlest, the smallest detail when you learned about the subject. This is the most complete knowledge, when there is even nothing left for you to learn about the subject

знать подробно - For me it means that you just went beyond the basic knowledge about the subject, developed an advanced understanding of it. In short you have a very good and deep knowledge about the subject, but that doesn't mean that there is nothing left for you to learn about it. See it's the main difference between words