A question about multisite Always On design
In traditional Always On Availability Groups, all replication traffic comes from the primary replica. If you have two replicas in a remote data center, they will both be updated from the primary replica, period. You can't do primary-to-secondary-to-secondary.

SQL Server 2016 introduced Distributed Availability Groups, which might satisfy your design goal. However, this is going to be much more complex, much less documented, and I wouldn't recommend doing this for your first AG.