Best way to flavour f2?

I recently finished the second fermentation to my kombucha and used homemade quince syrup as flavouring, but it didn’t carbonate at all after 2 days, so i just left it sealed in my shelf. After a week or two, when i remembered it existed, i checked on it again and it was quite fizzy and tasted nice! Just curious on how to flavour kombuchas, so i don’t over-sweeten them to not kill the yeast (methinks that was the problem)