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Do olympiad training and work through the art of problem solving.
Don't listen to the weak plebs telling you to read textbooks that you're going to read in a few months anyway.
It's more important that you're as sharp and strong and as good at raw problem solving as you possibly can be. That's what makes you actually good at maths, not simply how many theorems you've memorised like stamp-collecting.
An IMO medallist who gives up on maths after highschool and decides to study economics and make bank is always going to be stronger at maths than some mediocre pleb who can't solve hard problems, even if they have a PhD.