Is it worth completing all textbook questions, making summaries and flashcards over the summer holidays?
For maths in particular, if not completing all questions what would you recommend doing to get ahead??
Yes, I think it is definitely worth doing something during the holidays but I will say that it is definitely not 100% necessary to do so - you can still do well if you don't do anything so don't feel that there is pressure to do everything.
For maths, I think it would probably be the simplest to independently study since you can just read the textbook and watch videos online to learn and then complete the exercises following that.
Ultimately, I think having an in-depth understanding of a small part of the study design will be better than having a superficial understanding of the whole thing so I wouldn't be rushing for the sake of completing the course during the holidays.