We concluded that the 3-week time term was the best sprint duration for such teams. 2 weeks is too little to do the required regression testing, and 3 weeks is a minimum duration to ensure quality deliverables.

I would not go with longer sprint/release cycles because speed is a very significant part of Dynamics 365 implementation projects' value.

We start a sprint with the previously prepared scope.

Sprint Management for Dynamics 365 Projects - The Sprint Cycle

The technical team works on it for the first 2 weeks, until those features are deployed for stabilization.

The features are tested as they are developed by all interested parties. Three days before release, features should be finalized and deployed to a Staging environment for regression testing.