Hi I am the first batch stpm white mouse haha and I scored 4 flat, so i believe my opinion would be credible for you?
STPM is harder than SPM ofcoz, just like SPM is harder than PMR, but overall, the difficulty is still manageable.
your choice of subjects belongs to the physic package in my school, when it comes to applying for degree courses, it shouldnt be any problem unless in certain Biology-orientated subs.
In my opinion, PA is pretty much the easiest subject to get A if you have a good teacher. My teacher focus on broadening our knowledge of current world issue and national ideology and such. In simple words, my teacher shares alot on general knowledge, which is the purpose of PA, and teach us on how to write a good PA essay. Instead of those old school essay memorizing and plain old point discussion, we focus more on practice and grouping of similar topics and related points together. As for the elaboration, you should show your high order of thinking skill as per the MPM which is just plain bullshit. Just bring up some facts, for example the United Nation(UN) and talk abit on what they did, thats all. No hardcore memory work is necessary. As for sem 2, it will be graph drawing which is plain stupid and easy marks for you as long as you follow the guideline and have some tiniest bit of simple logic.
For maths, well it certainly scares me the most but I obtain a principle A for every paper haha and for my sem3, I only use 30minutes to complete it and exit the hall 1hour prior to the end time. Practices and comprehension is very important here. But mostly, its the effort that matters. Do alot of exam-style model paper will do. Its much more effective than tedious drill on each topic.
For chemistry, sem 1 you should focus on the technical rules and calculation. Its pretty simple but technical errors can get you no mark. EG: equations must be written clearly for each answer whenever necessary, decimal places rule which you should confirm with your teacher later, and abit of revision will do. Be careful in your calculation though, sometimes the question like to trick you with the physical states which may affect your quantity in calculation.
For physics, unfortunately Im a biology students, I may not be that suitable to answre you. However, my advise is make sure you understand the concept completely and write every single calculation step that you made while answering, focus on the units and most importantly, do alot of exercises

I may not take physics but just to let you know, Biology is much harder to get A than Physics, said by the stpm students though.
Lastly my general piece of advice:
1)Revise regularly, a little bit a day makes you go for a long long way.
2)Do alot of model papers before the actual exam.
3)Make sure you have the correct attitude in studying. Do not rely too much on tuition teachers and dont disrespect your teachers in class by doing your tuition work in school or whatever private institution you go for STPM
4) Dont be down just because you didnt do very well in sem1, theres always the luck factor and random ambiguity in the Malaysia general examination.
I did not take any tuition for the past 20 years, and Im not a study-grind freak, just manage your time well, spend an hour or so in after school revision, listen attentively in class and you shall do well in your exam

I believe the new STPM is far more easier than its precedents.
People who get A C B+ in their 3 papers can get an A as their overall grade too because of 2 words. Malaysia Boleh!