If you don't get on with the Bugaboo Ant, I would most certainly not look for an ultracompact. Even more so if your requirement is everyday use and a high seat capacity (and the seat needs to be large to accommodate a 25kg child! Even if a small buggy states a high weight capacity, it might not be enough! The Cosatto Whoosh being a living example of that, and yes, it is not an upright stroller at all).
For a tall toddler, do look for a larger seat surface buggy - something like a Baby Jogger City Mini 2 or a Baby Jogger City Mini GT2. These would be my first choices. A Thule Spring might work as well, or check the Uppababy Minu. You will need quality and a good brand to endure high weight, all kinds of use, and to have ample space for your big little one. 😊
P.S. If the stroller really really REALLY needs to be a small one, do check the Joolz Aer, its backrest is very high, considering.