Delete separate photos for HDR and panoramas, or not?

Something you could consider when thinking about a panorama: do the individual photos have value, meaning: if not, I would settle for the final panorama photo.

And furthermore: I don't know whether the technique of merging photos, panorama or HDR, could improve much further. That would mean that keeping them is pointless, because the end result is already there.

And finally: you had a goal in taking individual photos for HDR & panorama: that goal has been achieved, so what would you still want to do with the individual photos?

I keep them because it quite often happens that I want to merge those old ones again because the quality of the programs (photoshop) has improved so much... besides, insights and taste also change and that too can be a good reason to want to do it all over again and possibly with different techniques

Backing up your files (photos are on an external hard drive), which systems are worth considering? Part of an existing course or how can I find out more? Can you give advice?

I too am busy tidying up. Sometimes unfortunately too much and too quickly. Especially when I have made an edit that I have edited undesirably in a plug-in. I can no longer go back in the edit in LR. Some photos can work in a panorama of 5 photos, but later I think that 2 or 3 photos could also be nice. So last week I agreed with myself that those photos would get an X flag and the good ones a white one. Still more time to think, because the possibilities for editing photos keep growing and then it is wonderful to edit the older photos again.

Usually I keep a single photo from a panorama series that has a good composition in its own right. The ends usually go then.
Have found that the well-exposed photo from an HDR series usually shows a different color pattern than the composite result. I usually keep that one as well.
I usually make series of photos that are only meant to be viewed and not projected quite a bit smaller, which saves some space too.
But I can imagine that Nando has more shots than I do and therefore the erasing brush needs more hairs to erase.

In this age of AI, I find myself hesitating quite a bit about throwing away some panoramic images, especially if you also want to enter a photo in a competition now and then. The rules often say that you must always be able to prove that the end result consists only of photos taken by yourself. Well, if you then throw away the originals, you can create a problem for yourself and get disqualified. All because of AI and the possibilities of image enhance or image extender.
In other words, you will have to carefully consider what you do with each photo, throwing things away too quickly can cause problems later on. But you should certainly clean up regularly.

Photos that aren't worth the effort aren't used to make composite photos. They don't even belong in the selection, so they're thrown away.
The RAW files that are part of a composite photo, I keep. The reason is quite simple: when I started with digital photography, I had an Olympus XZ-1 and the RAW photos from it are still worth keeping. More than that: with DxO PhotoLab I can still get more out of them now than I could back then with RawTherapee. So my reason for keeping them is clear, although naturally the limit is set by the information actually available in the RAW file.

As far as I know, merging into a single DNG file is only possible in Lightroom. Since not everyone makes money from photos, that isn't a universal option. In that case, keeping the RAW files is wiser, especially since the stored information takes up just as much space.