If I keep up with the traffic and do not accelerate it unnecessarily, it would give around 13 with full AC. and if I overtake some cars and use accelerator to attain speeds wherever there is space then only 9-10km.
Having owned corolla too, I would suggest go for corolla if you can stretch your budget, or get 2020,2019 model used one(gli auto,altis1.6 both are very good for highway commutes), its far more quality build than yaris, and not to mention yaris suspension feels like honda. You would feel every bump inside. Even yaris's engine noise is too much, maybe because of cvt I notice that.
I am sure you will not regret that decision after taking a drive in Yaris especially CVT one.