Hey! I completed Psychology U3/4 in 2018 with a raw study score of 41. My SAC scores averaged from 75-95%. Just like yourself, I didn't do any 3/4 subjects in Year 11 either, and Psychology was of my 5 subjects too.
You're doing fantastic already by the sounds of it. I was a bit laid back when it came to exam revision, I probably began revision about a month before my exam? At the beginning of my revision I tried to complete practice exams all in one go, this may be an efficient method of study for some but it personally burnt me out. Trying to shove so much content into such a small amount of time didn't work for me.
Instead I began doing practice SACs for each topic. I would complete one for U3AOS1 with my notes beside me, and then would complete another one for U3AOS1 without my notes. I did maybe around 4-6 of these a week (2 for each area of study), 3 weeks before my exam (I know, should've studied sooner). I actually only completed 2 practice exams for psychology after finishing all those practice sacs and managed to come out with an A+ on the exam.
I'm sure I could've definitely benefited from completing more practice exams but I found working on the information in smaller chunks really benefited me and I wasn't as stressed as I wasn't looking at the "bigger picture" of everything I needed to know!
I know its easy to say, but try not to stress yourself out, you seem incredibly motivated and you will be fine
I hope this little insight on how I studied helped you out. Best of luck!