The first thing that came to mind was the
Monteray Booster. Covers all your points except doesn't harness, just uses the car seatbelt. They're on special $219 with free shipping.
Your best bet with harnessing over 18kg is the
Radian . It's more expensive at $519 (also free shipping) but it's hard to find boosters that harness past 18kg, all options are pricey.
ETA hmm, your DS might not be tall enough for the Monteray, you can try one at Baby on the Move if you have one near you.
Edited by arohanui