I don’t think there is any real tension here. TDD is a method for writing code. Private functions are a convention for maintaining code. If TDD helps you build out working implementations, then write it that way and then make the functions private later and remove the tests.
But in my view the value of TDD is primarily as a tool to help me think about the API (since writing the test requires defining the inputs outputs and name). dbg is way more useful for sanity checks that I’ve implemented something correctly, as you describe in your example.
