Hello Peter
The only way I found how to make this happen I found out the hard way some time ago 🙂
In my Video I made a Parent Track and 3 child tracks per screen. In the same way you started.
Then I made another Parent track and 3 child tracks, etc, etc
Just a matter of matching up when the First scroll get to the top of the screen you need to adjust the Second Parent to slide the image up the join the first, etc. and when you scroll back and the previous screen is fully show the need to adjust the roll back of the second Parent.
Not easy to explain. I will attach the Veg file without media if that also helps
As I only had a few bits of video I used just still picture to fill the project just to show you