Ehhhhh, what's your problem with booster seats? Those are just to get the shoulders into the design zone as needed for the typical 3-point seat belt.

It's based on size (shoulder height measured in sitting position) not age.

I think some 5-point seat belts allow a much wider design zone so won't need boosters by the time the children outgrow the front-facing bucket seat.

Kids want to get out of them into adult seats as soon as they can. Actual comfort not relevant.