I know this isn't the answer you're looking for, but it's all I have. When my company migrated from Exchange to O365 we used an outside consultant - and we're glad we did. It was not an easy process. They of course used 3rd party migration tools (don't know which) and although it was for fewer than 50 employees, it took a lot of planning on their part and the better part of a full day to execute the migration (and then some follow up to mitigate any minor issues that arose). And btw, our top folks are all IT people (Ex CIO of Fortune 100 companies & telecom professionals) - and we chose to "outsource" and make sure it was done right.
As I said, I know that doesn't answer your question directly. While we can offer some input for questions such as yours (and perhaps one of our other members can do better than I just did), forums such as Computer Hope are geared mainly toward the individual user as opposed to corporate issues. Wish I could be of more assistance.
