Update User/Customer object via API

Hello! I am looking for some best practices/guidance on how to best solve for the below:

I want to update the Zendesk User/Customer object via the API. The update will pull in data in custom fields, such as Subscription Plan (free/paid), Last Payment/Renewal Date, Link to Internal System to the user's profile. The purpose of this is to enable accurate reporting of metrics depending on Subscription plan & efficiency gains for agents. 

The reporting is done on the ticket level, so it will need to reflect the subscription plan of the user at the time the ticket was created. To achieve that I might add a tag to the ticket at the point of the ticket creation or update a custom ticket field. 

We got several millions of users, and there can be multiple updates on their Profile, which can pose challenges on how we handle the update via the API. We also have a Fivetran API connection as we are doing our reporting on Tableau. Here are my thoughts: 

• Solution 1: Import all customers and their data into Zendesk, and subsequently add new users as they get created in our system AND update existing users as and when any of the details above get changed. 
• Risk: I can easily see us reaching our API limits (700rpm), but also, not all user data imported will be usable/relevant

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• Solution 2: Import only Paid customers and their data into Zendesk, and subsequently add new users as they get created/upgrade in our system AND update existing users as and when any of the details above get changed. 
• Risk: The absence of a Free user might create confusion as to why there are no details in their profile, and missing out on efficiencies from giving agents the ability to just click on a link to view their 'internal' profile. 

Also, in the future we might want to add SSO on our Zendesk hosted help centre, using our app's login service - in case it has any bearing on what would be the best approach here. 

Has anyone done something similar, and how did you solve for this? 

Thanks a lot!