I do all of my code in notebooks. If your docstring is slow to read, it is a bad docstring. To be clear, a docstring is the thing you get when you have a function, you don't know what it does, and you press shift tab to bring up the brief description of what it does in a pop up (in jupyter lab). It should be the quickest, least error prone form of documentation because it is available on demand and is supposed to be short.

Using notebooks as functions is a bad idea. I think papermill is trash.

Notebooks are good for illustrating what you did and what you got. They are not good for illustrating how you did it.