Should I buy travel insurance before confirming my trip in September?

You should buy only after confirming the trip! there's no use buying it so early! You can even buy it a few days before the trip :)

If your trip isn’t confirm, I wouldn’t get a travel insurance especially if it doesn’t have a 100% refund policy.

Travel insurance usually have discounts almost all year round, so you would not be missing out on much! I would rather lose that 20% discount than my entire insurance premium if my trip isn’t confirmed.

Though I do believe there are some insurance in the market that allows you to change the dates of coverage, do check with your trusted financial advisor on that! Hope this helps! :)