I found this to be of great help:

1. The easiest thing to do is to set the VerticalOptions on the ScrollView to "StartAndExpand". This will trigger the legacy behavior for StackLayout and make it act as if it were a single-column Grid. (This is effectively what it's doing in Forms when it special-cases the "ScrollView inside StackLayout" behavior.)
2. [My prefered solution] Alternatively, replace the outer StackLayout with a Grid. This will constrain the ScrollView vertically and allow for scrolling. "But wait, I want to stack things above the ScrollView." Okay, divide your Grid up into two rows, and put a StackLayout in the first row source: https://github.com/dotnet/maui/issues/12452#issuecomment-1683054644