Tado can see a window opening by a drop in temperature, but then simply stops heating for a period of time because it can’t tell when the window closes. I know you said you didn’t want that, but isn’t it a way better option to put an actual door sensor on the window and act on that? Xiaomi has very small, good and cheap ones (either the Aquara or the Mi) which you can get for a nice price from Bangood or Aliexpress. Those sensors can be used for much more, e.g. alarms with the Heimdahl app.
It’s quite likely Tado won’t trigger window flows because you don’t have smart assist.