OK I assume that you had tested with a single stream (IO Meter).
The Standard Veeam Setting is a "Per Job" chain. That means you backup 20VMs with it´s 40 VM disks to a single backup file per restore point. So a single write stream is used.
If you enable "Per VM chain" at the Repository settings, for each VM a own backup chain is created at target storage. How many in parallel depends on the Proxy and Repository task count setting. So at least you will see 2-4 write Streams per Job.
That way you teaming will be better used as well.
If you then use Win2016 with ReFS 3.1 formated block volume (iSCSI) and it´s "Fast-Merge" processes, this would be the optimum for your storage.