Hi, I am a Durham natsci offer holder and am planning to go there in September.
Looking at Durham's transparency report for the 2022–23 cycle, this programme had a really high offer rate (97%) which is regardless of the subjects you choose to appply for. Im sure if you get that you will absolutely get an offer since actual grades are pretty much always valued over predicted grades.
Reading your post, instead of a gap year? perhaps you could still try to apply there via UCAS extra? This year I essentially got 5/5 rejections and firmed an alternative offer given to me by Bristol. I ended up calling them and UCAS to say I wanted to withdraw then applied to Durham's natsci course via Extra. They gave me an offer 2 days ago, a week after I applied, and even gave me a contextual offer (more likely for personal reasons) so I only need to achieve AAB to go there. I'm sure they still have vacancies so maybe you could attempt the same before taking a gap year if you dont know what you are going to do with it.
By the way, what subjects are you planning to do at Durham? I'm going to be doing Computer Science and Mathematics