Creating a azure subscription for a client - but not manage them. What's the best approach for this?

Hello -

I work for a consulatancy where we develop Data Platform on Synapse and Fabric for few small banks using Core Banking systems as the source of data.

We develop and deploy all codes to banks env (DEV, UAT and Prod) but we are not responsible for managing the costs of these resources.

Different banks have their own IT team which creates the Azure Subscription and some are administrted through other companies.

The current bank does not have an idea how to move to Azure and wantsour advise.

I need to know few things:

1. Who should create the tenant?
2. What type of subscription to create (we need nonprod end env and prod env).
3. what to suggest to the bank ? pay-as-you-go or Reserved? I need to understand how reserved work?
4. Again we are only the advisory here and will not be involved on payment or anything.

My job is helping them making the best agreement with MSFT to move in Azure Cloud.

Thanks

Xhev