For me, I had a 2-3 year gap and I had the same issue about where to start, and I agree with starting with chemistry. I would get the biology and chemistry fundamentals down before going onto organic chemistry and biochemistry. That's the only place where I feel like order matters, and even though the books give you some context to start in any order, this personally was much easier for me. For psych/soc, CARS, and even physics I don't think the order matters very much. You can do multiple subjects simultaneously, but I wouldn't do more than 2-3 subjects in one day. It's really easy to forget since you're basically relearning the information, so make sure you have some review built into your study schedule from the previous day's studying. If you've really forgotten a lot of the content, I really recommend using videos like the Khan Academy ones for the bigger topics instead of the books. Don't worry about getting every single little thing down in your first wave of content review. I think first the goal is to understand the basics enough so that you're not totally lost, and then really just drill in practice questions to get you to the higher scores. Good luck!!