I know the size the photographs gets substantially bigger with the upgrade. I currently have 400 or so units free out of 500. Would it make sense to increase the amount available ram to 1000 (which can get expensive) or just change my habits and offload more Applications or photographs into the cloud.
I would always try to keep less than 80% of my memory occupied for good housekeeping. If I buy a new iPhone, and I am already tight on storage, I would definitely choose the next storage tier when spec’ing the new model. You, however, seem to be on the other end of the spectrum.
If you’ve got 400 or 500 free…no real reason to upgrade. Yes, the 15 can take 48MP shots but only in a limited number of situations and focal lengths. My XS Max has 110 free of 256 so I’ll probably upgrade to 512 myself (unless that’s the minimum) but then I’m over 50% used and you’re at 20%. If you’ve got optimize phone storage turned on it will likely not ever be an issue for you I wouldn’t think.