Migration solves a location. Modernization solves a business constraint. Confusing them creates technically successful projects that leave the difficulty of change untouched.
The right starting point is not which cloud service to use, but what operational change the application must enable and which risk must be decoupled.
A sound path separates portfolio decisions, target architecture, migration waves, delivery modernization and stabilization. Every wave should leave a usable improvement and evidence for the next decision.