There is no way to trigger behavior formulas based on the current item on form load when using SharePoint integration (Ex: Custom SharePoint List Item Edit form). Note that the SharePointIntegration OnNew, OnEdit formulas do not allow for this. Ideally, Data Cards would have an OnInitialized or OnLoad function. The individual field controls (text, dropdown, radio, etc.) could also benefit from the OnInitialized formula as well, since OnSelect and OnChanged do not trigger.
The proposed behavior function needs to be able to access the Data Card objects and ThisItem after the data cards are initialized so we can programmatically set properties or variables.
ListA Edit Item Form PowerApp
On form load, code needs to run to dynamically set fields based on another field's current value using basic custom business logic.
Ex: If fieldA.Value = Option2(select(ControlB)) and set variableA to true. In this scenario, variableA is used in several other fields to dynamically configure properties and is based on FieldA.Value and ControlB performs some custom business logic.