Re: The ONE thing I can not figure out!
With those stats I stand by the TF. They have high slots so good for tall LO. They rf to 35lb which should be fine for you. With a thin child you could get over 3 easily. You can ff to 65lb and you would likely feel ready for a booster at that point. They are readily available for between $150 and $200. I really like the feature of being able to remove the headrest up to 22lbs.
Just in case.