Booster help for 38lb 6 yo

[turtledove]

I'm having a hard time figuring out what to do for my 6 year old daughter this year. For background, we go to private school and carpool with another family. The other family has one child a 12 year old- they are far out of the carseat stage. They will need a carseat that is lightweight and easy to move out of the backseat (no permanently installed carseat). If my daughter was two pounds heavier, I would have a ton of options, but she is a featherweight (my 3 yo is only two pounds lighter and he isn't exactly big). I see the Aidia Scout Backless Booster is approved for 30lbs. Does anyone know anything about this seat? Any other recommendations? She is still in a 5 pt harness in our car. The carpool will be around 5 times a week, and it is 10.2 miles door to door. They could be traveling on highway or backroads depending on traffic.

ETA- she is 44.5 inches tall

TIA!