Thanks Robert. That's very helpful. I will do some experiments with importing the code into a notebook. But, on the basis of your thoughts, it sounds as though the process of editing the code in IntelliJ is really just a one-way process. I.e. once I have decided that my programming task demands a more capable editor, I won't be going to back to Mathematica, except to run the code.

For large code development, I'd suggest using an IDE that supports it. AFAIK, there isn't one that works with notebooks, so this is sort of a half-answer: Someone has written a nice IDE for working with packages (".m"/".wl" files):

https://wlplugin.halirutan.de

There's also Wolfram Workbench.

You can save a .nb file to a .wl if you change all the input cells to be initialization cells; then Save As .wl. Can be a bit of a pain, though. Not sure if you can convert back to a .nb file.

Not a quick solution to your problem -- sorry about that.