It depends on the larger context.
I've read your example over and over and don't find it confusing. But the word "therefore" is odd. It implies that there's already a discussion immediately before this sentence about those courses. If that's the case, then it would redundant to restate the information about the courses and it would, therefore, appear cumbersome.
If you said "It turns out" instead of "therefore" the long sentence might fit better. But it would still depend on what came before the sentence.
Yes, in general you want to avoid long subjects like that but, if they're necessary, it's okay to use them. The fact that you already took the courses is quite relevant and the fact that they're PhD level courses that you took before you were a PhD student (maybe) is reasonable information. It's just all information that doesn't have to be in the same sentence with the most important part: the utility of courses from one field for another.
So it's less about specific rules and more about the overall readability of your work.