In my experience, don't get too caught up too much streams. You only use one service account for admin. Access Nodes use App Registrations using OAuth2 and it's the number of App Registrations the provide the throughput. The App Registrations will balance between the Access Nodes.

You can no longer create sub-clients for the Mailbox agent (besides the default)  

Is there a particular reason you are attempting to create them?  For the Classic Mailbox agent the split was to help with performance but since we can add more streams to the new Mailbox client it is no longer a necessity. 

@Chris More performance and diferent schedules for the subclients. This customer has about 4000 mailboxes. 
If you have one access node you can have  a max of 10 streams? (books online)

We also want to implement 3 access nodes
1 access node 10 streams 5 service accounts (books online)
2 access nodes 10 streams 6 service accounts

Correct!  Service accounts are really only used for the discovery process to access Azure AD or Local AD depending on your configuration.  The backup should use registered applications with your azure portal to perform the backups.  I would setup the three access nodes and run a chunk of mailboxes and see what we get from the performance, once we get a bulk of the data backed up its just incremental moving forward.