120 wpm using mostly thumb and index fingers sounds insane to me. I type using standard touch typing and can only get to about 60 wpm. I've always been a bit of a slow typist. Can I ask how you measured the 120 wpm?

Just the standard online typing tests you can find by googling. In fairness, 120wpm is a maximum. I'm often slower than that. But always over 100.

Edit: For those who are curious, I took this test: https://www.typing.com/student/typing-test/1-minute and got 116 wpm at 99% accuracy.

At https://www.typingtest.com/benchmark.php over two minutes, 117 wpm at 97% accuracy.