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.

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.

Evaluate this in a notebook.

SetOptions[InputNotebook[], AutoGeneratedPackage -> Automatic]

Whenever the notebook is saved, a corresponding .m file is saved with all of the initialization cell definitions. The convenience of working with a notebook, and generating code for re-use in other notebooks.

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.