So like, my perspective as someone who did methods 3/4 and specialist 3/4 at the same time. I'm not gonna say I'm right and others are wrong, but I find this posts are constantly occupied by people who did methods 3/4 and THEN did specialist 3/4 the next year - and they always say the same thing. "I think doing methods first really helped with specialist", "I definitely think I scored better in specialist because I'd already done methods", "I don't know how I'd feel having to study for 2 maths subjects at the same time, doing them subsequently definitely worked for me".
As someone who did "2 maths subjects at the same time" - tbh it didn't feel like that at all. By the time I was covering topics in methods, I'd already seen them and understood them in specialist. Until probability, there was nothing methods could throw at me that I hadn't already seen and understood from before. It didn't feel like studying 2 subjects, I was studying 1.5 subjects - and tbh, doing specialist at the same time I reckon incredibly inflated my methods scores. Like, maybe specialist will be slightly easier if you do methods first, but you'll definitely be able to do less study for methods and do better in methods if you do specialist at the same time.