Hello!
I think the Valco Snap 4 is a relatively spacious stroller and should be able to handle a two and half years old child (in general, the child doesn't need as much space around him/her in the seat, and a slightly tight fit is actually not a bad thing, especially in winter). If you are so happy with your Snap, maybe you'd consider the higher version - the Valco Snap Trend Tailormade. The improved Trend Tailormade is similar to the original Snap, but offers more space and comes with some extra functions.
On the contrary, I find the Britax B-Agile 4 Plus not-too-good a choice. Yes, it may offer a bit more space - although not necessarily, I mean not in the Agile 4 Plus version. It comes with a much lower-positioned handlebar, less suspension, and a center of gravity not suitable for everyone. Overall, I think the Agile 4 Plus is a bit outdated, and nowadays, there are much better strollers to choose from, including your Snap.
The Britax B-Motion Plus offers more space for sure. Most likely issues you'd have with it are:
Size/weight. This stroller is much heavier compared to Valco strollers, bulky, robust, and doesn’t offer that much compact fold, which you probably got to expect if you are used to the Valco Snap.
The seat is slightly reclined even in the upright position. It can be improved by adding a strap to the backrest, but certainly worth to know beforehand. Some mums are fine with its center of gravity, yet some mums are unable even to push the stroller for longer periods of time. Therefore, I recommend testing the stroller before purchasing it. It is the only opportunity to see for yourself whether you can manage the stroller with your son on board (especially when it comes to lifting the stroller over a curb, which surely try if going to look at it in some shop).
My advice would, however, be to stick with the Snap. After owning the Snap, I doubt that I would swap for Britax or any other significantly heavier stroller. I would probably consider changing it for something lightweight and more compact, such as the wider - super spacious Oyster Zero from Babystyle. From those heavier strollers, I could possibly suggest the Moon Jet-R.