My kid was also 52" at 6 yrs old, but she didn't quite make it to 50# 'til she turned 7 -- she carries her height in her legs & outgrew the Safeguard Go when she was 5. She also has some undefined attention and/or sensory issues, which made booster use a bit more difficult for her than other kids, but usually only when there's another child sitting next to her (otherwise, she maintains proper positioning by keeping herself entertained with finger puppets, books or her Gameboy).
Unless you've turned off signatures via your UserCP, you can see how she rides below
She prefers the Husky/Regent over the Monterey, but she prefers the Monterey over the vest (which we use for taxis/travel). We both also prefer the Monterey over all the other boosters she has had (Parkway, Compass B510 & Turbo) for it's comfort, size & sturdiness.... She likes the snugness of the harness, but she sweats in the vest. In a booster, she always prefers to engage the
Switchable Locking Retractor (by pulling the shoulder belt all the way out, slowly, letting it ratchet back in) which keeps her in place, providing that desired snugness.
Locking the shoulder belt is merely an optional passive reminder for wiggly kids to sit still & most kids respond appropriately with a warning: "sit back stright or I'll lock you in!"
Boosters are designed specifically for kids your child's age & size, so if your child doesn't have any behavior/attention/sensory issues, at this age & in that vehicle, a booster without the shoulder belt being locked is a safe option :thumbsup: Kiddo should be able to learn quickly not to lean forward or sideways, etc.