Creating resource mailbox from a federated AD account

I'm working on a script to generate resource mailboxes for my organization.  One of the requirements of this script is that the resources must be associated with a federated Active Directory account for a variety of local account management concerns.

The current process we are using is as follows:

1. Create the AD account
2. Wait for the periodic DirSync process to complete
3. License the account with an Exchange Online license (creates mailbox)
4. Convert the mailbox to a resource with Set-Mailbox <mailbox> -Room | -Equipment
5. Configure the resource calendar processing as appropriate

Based on my understanding, resource mailboxes do not require an active Office 365 user license (and all of the AD accounts are configured as disabled with no password, as are on-premise Exchange resource accounts).  As such, I would like to be able to collapse steps 3 and 4 and just create a resource mailbox directly.

So far none of the parameter combinations I have tried with New-Mailbox have worked.  Is anyone else doing something similar to this?  It isn't a significant issue to license these accounts, if necessary or if my understanding is incorrect; I am just trying to create the most efficient process/script possible to hand off to my ops team.

Thanks!