Absolutely keep all the lights. As seen recently with the development of the NormalizedScaleGradient script in PixInsight, improved ways to do even the initial steps can come up at any time.
The lights will be most of the data so how much you keep from the other items you listed is a matter of taste. I don't normally keep individual darks, flat darks and flats, just the master dark and master flat. You can keep the master light just after WBPP, why not, it's just one file and it will save you some time, should you want to re-process starting with it.
You can keep the projects as well, especially initially. I would expect that after a few years, one would start from fresh rather than jumping in the middle of an older project so you can prune them then, if you want, but it's up to you.
As I am starting to amass a lot of images files, I am trying to understand which files I should be keeping in case I want to return to the target sometime in the future.
- Do I store all lights?
- All calibration frames?
- Master integration files from WBPP only?
- PI projects?
I do not want to store locally, it will all be in the cloud. Its not really feasible to store everything (storage/cost).