B.H.
Most carseats that convert to boosters make horrible boosters and don't allow the seatbelt to fit properly.
I would recommend a carseat with a higher harness weight. It's safer to hold off on a booster for as long as possible. There are alot of seats with higher weight limits Graco Nautilus, Britax Marathon etc... They go way above the traditional 40lb. weight limit.
My daughter is almost 7 years old, and is still in a 5pt. harness.She still fits in a Britax Marathon but we like the extra room of the Britax Regent. I hate boosters. We have a booster but use it only for a back up seat.
You should try this website it's a carseat forum where other mom's and carseat techs can tell you exactly what would be the best for your daughter and your family.
Check out this quick video it was a eye opener for me and had me in tears.
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