Insurance is always best purchased as soon as possible once you’ve bought something. If you’ve already booked your venue or any vendors, then you want it right away. This is because if a company you’ve booked goes out of business and you have insurance the insurance will help cover your costs. I think there are still just two companies offering insurance at the moment, cover my wedding and wed insure, but not entirely sure on that and worth doing some research. Make sure you read the sections of cover and understand what will/won’t be covered before you purchase to get a plan that suits your needs.
Also, if you can find out their claims payout percentage that would be useful info for you. I don’t know if it’s easily accessible though. I work for a pet insurance company which pays 98% of claims so obviously that’s used in our marketing but idk if the information would be in the public domain otherwise.