I would do it that way.
Set both anchors of the background ImageView to BOTH, this you already have.
Add a transparent Panel and move all views onto this Panel and center it on the screen in the Script.
In Scripts-General add:
AutoScaleRate(0.5)
AutoScaleAll
Panel1.HorizontalCenter= 50%x
Panel1.VerticalCenter = 50%y
Then you can play with the AutoScaleRate.
With AutoScaleRate(0.7) you fill almost an iPad screen.
When you change the AutoScaleRate click on Run (F5) to see the result in the Anstract Designer.
Attached your modified project, I added a new layout file keeping yours for comparison.