Having done several Exchange migrations myself: You should read these documents. One important thing to do is to create a common name space which does not refer to server names, such as mail.domain.no.
Migrating to Exchange Server 2016
https://practical365.com/exchange-server/migrating-exchange-server-2016/
There's an excellent 3.15 hours course at Pluralsight (sign up for a trail)
https://app.pluralsight.com/library/courses/migrating-exchange-server-2016/description
Paul Cunningham has a complete course on Exchange 2016 as well. Cunningham is one of the world's leading Exchange experts. This will answer your questions on DAGs as well. To give you a hint: Usually you need to do nothing. With DAGs in use, they are never completely in sync. However, there are transport resiliency features: Safety Net and Shadow Redundancy
https://video2.skills-academy.com/en-us/exchange/mail-flow/transport-high-availability/safety-net?view=exchserver-2019
https://video2.skills-academy.com/en-us/exchange/mail-flow/transport-high-availability/shadow-redundancy?view=exchserver-2019