That's a very good question. Thank you.
In my personal experience when we did this in the company I was involved with, the biggest issue we struggled with was trying to create one massive project versus breaking it into multiple projects. So you could not track the progress and you could not see the benefits at each stage of the way. If you keep it as one very large project, it's very hard to have an exact start date and end date and to have the benefits as planned by the project managers coming through to the senior leadership.
Our approach from an experience point of view was to break up the large project into multiple projects and hold accountable the people who are doing it for delivering the benefits they put out along the way. Otherwise, you're not going to see it.