I would recommend you do methods 3/4 next year if you are willing to go over the essential unit 1/2 knowledge these holidays, it sounds like you will probably need it for uni so it will be worth you time. Since there is very little 'new' stuff in 3/4 as opposed to 1/2, apart from a few new rules and harder questions (not greatly harder though), then you could easily get a 25 raw IF from day 1 next year to the end of the year you keep up to date and make good notes after each topic.
It's very important, IMO, to make detailed notes after each topic, it means you don't have to make them at the end of the year when you should be doing practice exams and means if you forget a topic you don't have to 'relearn' it as such, but just skim over your notes and it will immediately come back to you because you have written them. By the way, by 'detailed notes' I don't mean taking neat notes from your teachers white board in each class, I mean after each topic, go through your text book and other resources such as A+ notes and summarize the topic in a way you understand it. You may not like this strategy and that's fine as everyone learns differently, but for me this has got me from a D~C average in year 10 advanced general maths to a B+ ~ A in year 11 Methods, to an A+ in specialist maths in year 12

If you feel you're a confident maths student ~ B average ~ and you are willing to put in the work and time that has been suggested from people here, then I'm confident you will do very well

If you decide to do methods next year, then I would highly recommend you be able to understand and do the following things before you start units 3/4:
1: Basic algebra skills; rearranging equations and inequalities, solving them and sketching equations. This includes linear (

), quadratic (

) and cubic functions (

) (And maybe quartic though I've never been asked about them in year 12). And be a professional at solving simultaneous equations as they show up in EVERY area of study next year, they are not that hard to learn, but they are essential. This stuff is not really taught in year 12, there is about 2 exercises on it in the first chapter, but most of it is assumed, so this is by far the most important knowledge.
2: Know basic algebra around Logs and Exponents; Learn log and Exponent laws, learn how to solve them and sketch them. This is a pretty straight forward topic and is taught again in year 12 in detail. Just know your exponent and Log Laws and how to apply them to basic log and exponent equations and how to sketch graphs. eg
)
.
3: Circular functions; Learn the exact values (Important), Learn how to solve and sketch the basic Sine and Cosine graphs, don't worry about translated ones. Eg
)
,
)
,
+2)
etc. Don't bother with
))
etc. This whole topic is covered again in detail in 3/4, so just make sure you can do that basic stuff and understand it.
4: Have a good look at the calculus section; understand how to differentiate and anti-differentiate Linear, Quadratic and cubic functions. If you feel confident with that, have a look at the applications of differentiation and anti-differentiation as there is a large amount of this in 3/4.
5: Finally have a brief look at probability BASICS, ignore combinations ect. Prob in 3/4 is very different to 1/2 and IMO a lot easier in 3/4. You could probably get away with ignoring 1/2 probability.
Hope this helps, it's hard to say exactly how hard it will be for you, or how much time you will have to spend doing this stuff because it is different for everyone. So instead I'm showing you what you would have to know to get a decent score in methods (IMO). And given that next year you won't be doing 'the Asian 5', you will probably have more free time to study this
