While Britax seats are expensive, one great benefit is how easy it is to install them RF in small cars. We easily got a Marathon and Boulevard RF for DS in small 2-door cars. An older child, like yours, can ride at a more upright angle, so that the seat takes up less front-to-back room. The new ones have 40 pound RF limits, though most kids will outgrow it RF by height before weight.
The Complete Air is very tall, but installs quite upright without a hassle, so you might have luck with that one too. Also a 40 pound RF limit, but a taller shell allows kids to actually get to 40 pounds before outgrowing it!
The Coccoro is super cute and compact, but it is small, with a 33 pound RF limit and a short shell. It might not be the best, since you wouldn't get a lot of RF time left, and you'd likely have to buy another FF harnessed seat since DS would outgrow it before being ready for a booster.
What about checking out a MyRide? 40 pound RF limit, shell height is in between the Britax and Complete Air.
Do you have a Babies R Us nearby? They'll let you take the seats to your car to see if they fit! (Then, find a better price and order your seats online