I use the online broker insure my trip dot com. You can normally insure everything at once cruise, hotel, air, etc and virtually up till the day of travel. You can also add to your insurance if you add items to your trip. The booking date, in my experience anyhow, normally only impacts things like cancel for preexisting conditions.
So, I’m not clear why you are saying you can’t insure future flights, etc Even if you already purchased your insurance or need to change anything like dates you just notify the insurers and if necessary they reprice and either charge you the difference or refund the difference. You can even move insurance to a different trip sometimes - I won’t bog you down with details.