Cutover Migration from EX2010 to O365

Question

Starting this weekend, we are going to be performing a cutover migration from Exchange 2010 to Office 365. To give an idea of the size of the current email system, we have around 90-100 users and a total of 400GB of data and approx. 2,000,000 items. Average
mailbox has 19,000 items at 4GB. The cutover migration will be done using an Outlook Anywhere endpoint. The internet connection from where the migration will take place is 20mbps download and 4mbps upload.
We were never expecting the migration to be quick, given the amount of data, but after some simulations and calculations we have found that, best case scenario, the initial synchronization phase will take 7 days. 
Our major concern is use of upload bandwidth during the initial sync and how it will bog everything down from a user's perspective, not just regarding browsing performance but mainly because on the same internet connection as the Exchange they have a site to
site VPN with another branch. We were therefore thinking of using the migration batch's start and stop functions to control the migration to happen only during out-of-office hours. Naturally the migration will take much longer to complete overall but the top
management prefer this than having 7 days of nearly unusable network performance.
This raises three questions:
Is the start/stop approach to the initial synchronization phase a good idea?
This is a problem I'm sure most companies face when migrating to Office 365. What approach do companies normally take to reduce the effects on the production environment?
What would be the best approach in such a situation, more simultaneous mailbox transfers (e.g. 10 mailboxes) to speed up the sync but bogging down the network, or less simultaneous mailbox transfers (e.g. 1 or 2) which might slow down overall migration but
hopefully free up some upload bandwidth?

Answer 1

As per your description, I would suggest you to start from here :
https://support.office.com/en-us/article/What-you-need-to-know-about-a-cutover-email-migration-to-Office-365-961978ef-f434-472d-a811-1801733869da
This covers almost all the required aspects while need to do cutover migration from Exchange 2010 to Office 365.
You should note that a maximum of 2,000 mailboxes can be migrated to Office 365 by using a cutover Exchange migration.
You may give a short span of time to this another informative reference :
https://support.office.com/en-nz/article/Perform-a-cutover-migration-of-email-to-Office-365-9496e93c-1e59-41a8-9bb3-6e8df0cd81b4
Also, Exchange server deployment assistant is your friend that is available from Microsoft team and provides step-wise assistance when performing cutover migration.
As an alternative, if you wish to accomplish this task automatically, you may also consider on this automated solution (http://www.exchangemigrationtool.com/)that could be a good approach for you to get
this job done without any downtime(that's the main concern of most of administrators).