Since Gdk 3.18, swipe gestures have been supported that give us proper event phases (as opposed to just continue and end). Given we ask for a minimum of 3.22, we should take advantage of these to provide proper event phases, which would help us improve scrolling behaviour.
Note that for scrolling begin events are synthesized with the first update event, and on GTK level - not even libinput. So I don't think it would improve much in practice.
This should get improved a lot when GTK gets hold gesture support though - then it should be possible to stop kinetic scrolling as soon as fingers are down instead when they are first moved.