We had the same prob and went for maxi rider. DD was over the top of the seat at 2.
The maxi rider has both the harness and the seatbelt options. We only went to the seatbelt a few months ago as after 20kgs the 5point harness isn't guaranteed safe. It's also still bolted to the car which as someone who has been to many many prangs in my time, I far prefer.