In the US, I believe maths is taught in a bit of a different way, so it's split up into e.g. algebra, geometry, etc. My understanding is that if a student e.g. passes geometry I but fails algebra I, in some schools they can just retake the bit they struggled with the next year. Equally, I believe they can follow different modular paths based on what they want to do e.g. chosing pre-calc or statistics type modules.
I'm not sure if this is the case in all schools or all states, but picking, and choosing and repeating modules could be a good model for post-16 maths. I believe Core Maths offers 2 pathways for students (quantitative vs statistical) which is a good start.
I agree, FWIW, a lot of students come to secondary school with low maths confidence, and gaps in their learning, which does really impede them. If there was a system where they could progress at their own pace and attend mixed age classes that might really help them BUT that would be a radical departure from the current UK system.
Core maths does a lot of things people on this thread consider useful, including looking at financial problems, and teaching things like using spreadsheets for maths (which is a useful skill a lot of students don't have). And it wouldn't take up a full A-level slot.
But, to offer it successfully, post 16 would need more maths teachers and better funding.