Term to describe paradox where those with less subject matter expertise can sometimes make better teachers?
This is called the curse of knowledge. From Wikipedia:

The curse of knowledge is a cognitive bias that occurs when an individual, communicating with other individuals, unknowingly assumes that the others have the background to understand. [...]
For example, in a classroom setting, teachers have difficulty teaching novices because they cannot put themselves in the position of the student. A brilliant professor might no longer remember the difficulties that a young student encounters when learning a new subject.

Writer and organizational psychologist Adam Grant nicely illustrates the concept in this article:

Two decades ago, I arrived at Harvard as an undergraduate excited to soak up the brilliance of professors who had won Nobels and Pulitzers. But by the end of the first month of my freshman year, it was clear that these world-class experts were my worst teachers. My distinguished art history professor raved about Michelangelo’s pietra serena molding but didn’t articulate why it was significant. My renowned astrophysics professor taught us how the universe seemed to be expanding, but never bothered to explain what it was expanding into (still waiting for someone to demystify that one).
It wasn’t that they didn’t care about teaching. It was that they knew too much about their subject, and had mastered it too long ago, to relate to my ignorance about it. Social scientists call it the curse of knowledge. As the psychologist Sian Beilock, now the president of Barnard College, writes, “As you get better and better at what you do, your ability to communicate your understanding or to help others learn that skill often gets worse and worse.”