Heya, Charlotte,
Watch out as you want something smaller than you have while it should be less stiff (= suspension) and roomy for a tall almost 2yo kid. Which normally means increase in size.
I personally would go for a Baby Jogger City Mini GT2 with most large children, but it is not a budget model. Also, not really that smaller than the one you have. From smaller ones, the only few that have a taller backrest are the Bugaboo Butterfly, Recaro Lexa or the Joolz Aer, though they won't be more terrain capable and they'll be narrower (and again not affordable, as the better models are always more expensive).
I personally would go for something like the Oyster Zero in terms of suspension + space, though it won't be smaller than your B-Agile M. Still, a more comfy one for sure. You do have to think about what you need (e.g. smaller size to be set as the priority), and the other features - the price, the seat space... will have to be a compromise.