Car seat help for tall child.

[Fuckingfuckssake]

Obviously posting here for traffic, I have a very tall but narrow DC, 9, at the very top of a high back booster but needs support still to position seatbelt safely at pelvis and shoulder, seems to be the depth of the seat as much as the height, does anyone have any recommendations please? We currently have a Cybex and the Britax Romer kidfix but neither are a good fit, seat only comes to mid thigh. Thank you for any advice.

My 9yo was getting uncomfortable, and we realised she is easily as tall as some adults who drive a car and have dispensed with the car seat altogether. She is over the heght limit for the boosters and seats we had so it is worth checking.
We have a special shoulder adaptor that helps the shoulder strap for her comfort.

Which Kidfix do you have? The 2 series were quite short in the seat pan but the Kidfix 3 are longer.

Joie Traver (Or Halfords Transfix I think it's called is the same seat) and Duallo have an extendable leg rest which comes out further.

Can you take him to a shop and try them all out? Also maybe a backless booster would be deeper? The backless ones can be a good solution when they get too tall for a high back but still need some adjustment. Not as safe, but better than nothing.