Do you know if her regent is installed with or without the recline bar?
If it is, how much of a lean is there to the installed seat?
That single item would make a difference, imo. DD's over 54" now & 8,5 years old, yet still fits under the top slot in her regent because it's not installed with a recline bar. She's also short waisted, but I suspect that the non-use of the recline bar has made this so long lived. (Her seat was made the month before use was made mandatory)
I know that other kids have long outgrown theirs because the recline of the seat put them over the limits. A more upright position sets them lower in the seat.
So based on how it's installed & torso height in combination would be the deciding factor on booster seats.
I can't give you any recommendations. I had her in a Parkway booster for use in friends' vehicles from 4-5 years old. After that, it didn't provide enough leg support. She was cramped and uncomfortable.
I jumped right to the largest seat I could obtain at the time for a dedicated booster. When she's not in her Regent, she's in a Monterey. With or without the back installed, it works. She's lean, leggy, and it fits her very very well.