I'm facing the same issue it started with my photo app growing up in space aside with system data. tried every thing and got in touch with apple support but all they do is simple things that I did many times but the problem still the same. I'm using iPhone 13 pro, IOS 16.0.2 I guess it is a problem caused by IOS 15 and they still can't fix it. Now I can't backup my device to a PC or Mac, can't move my photos to icloud of any other storage service. Every night i plug the device to a power source i woke up in the morning and find a message on the screen saying the storage is full. Apple support guys keep saying it is a simple thing and can be fixed easy and quick but they couldn't solve it yet

I guess they really don't have a solution for it yet. having the iPhone useless every now and then is more than annoying and as you said it is time to switch, apple is now what is used to be and the iphone is not the same

I'm seeking help here after trying many things I'm using iphone 13pro 356 GB IOS16.0.2