So glad you enjoyed it!
The main key to getting an S is all the stickers, each sticker is 500pts and the All Stickers Bonus is another 500pts, so if you get all of them that’s 2000pts by itself. Then it’s mostly about getting onto the grind rails as early as possible so you’re racking up the grind points and getting more chances for the balance minigame. Best of luck if you try it again, S is at 4000pts so you weren’t far off!