My advice would be to just start with FP1/FM1 and if you find you're struggling with one of them, pick up FS1 or D1 as a self-study. (or maybe FP2/FM2, though you might struggle to get teacher support with FP2 and will have a bit less practice material)
If you're good at physics and quite liked the mechanics in that, then there's no reason to think you'd dislike FM1. FS1 is a bit trickier, I'd say you'd only get a good indication of whether you like it when you hit hypothesis testing in AS (single) maths. The exam I found was fairly predictable (applying the same methods to different numbers) and similar to the textbook.