retaking year 12

I’m currently a year 12 student doing physics, chemistry, geography and welshbacc. I want to do astrophysics or something along those lines in university, but very few offer the course without a -level maths. I also fear I’ll struggle even if I do get in. Should I just apply to the few courses that don’t require a level maths, or retake year 12 so that I have a wider variety of uni and course choices?

My rule of thumb is to always do yourself justice and meet necessary requirements to widen your options as much as you can before settling. I would either do A Level Maths on the side, drop one of the nonessential subjects, resit the year, do a gap year to include A Level Maths as an extra subject.
You always have the option to do A Level Maths on the side at any point in time. For all UK exam boards (as well as international exam boards I think), the A Level doesn't require any coursework. In other words, you can just sit the exams as a private candidate.
The other key A Level for astrophysics is physics.
I'd be curious to know which unis offer astrophysics without needing A Level Maths, since many of them would require both physics and maths, and even if you miss out on one of them they would still require maths over physics.