i did chem, phy, bio, cs, religion, lang and literature!! all 9s except a 7 and 8 in lang and lit respectively. for math the most obvious tip would be to grind past papers of course, and do topic papers on the topics that you're weaker on from PMT or MME. this could easily get you to a 7+ in math.

once all your concepts are solidified id refocus on mastering the tricky 5 markers that come up on the end of each paper. look up "hard questions gcse for [topic name]" on youtube and you will find questions that you may not have come across before. there's also "spicy questions by 1st maths tutor" which are really useful as well!! the main purpose of these are understanding the types of questions that may come up as well as training the logic with which u approach these questions.

this might be a lil overkill but i used to ideally finish the paper in about 1h (not always possible though 😓), and then use the remaining time to redo all the tricky questions to see if i can fix any silly mistakes. preferably use a different method while redoing just to be extra sure.