Still booster. All the time. 30 mph is super severe. It's why carseats are tested to those speeds globally. If you impact above 30 mph (most crashes have some element of getting out of the way or starting to brake, or the cars are moving in the same direction) your chances aren't great no matter what. So just using a booster over 30 mph is like boostering for the top 10% of crashes only. The other 90% can still cause lifelong changes.
Most crashes don't occur on the highways where people are going in straight lines and traffic is separated and all going in the same direction. Most occur on city streets, near home.
My older was boostered until 12.5. She knew it was for the best. I think she was happy to be out, but she didn't ask until it was time.
Wendy