Storing contacts of different tenant members in a tenant level contact book

Question

We are currently working on this scenario: Given a tenant with multiple members, we want to import contacts of each of them from their google accounts and then store them in a shared tenant level pool in M365 address book.

We need help in the latter part - storing contacts in a tenant level pool. We have the following queries:

1. We searched for graph APIs to import contacts in an account. However, we could only find APIs that modify contact in a member's personal account and none for modifying contacts on a tenant level. Is there any graph API available to do so?
2. Another solution is for an app to get read-write permission across all users (https://learn.microsoft.com/en-us/graph/permissions-reference#application-permissions-19). If we can get tenant admin user id from any API, we can always save contacts to that account and retrieve from the same location. Is there any API to get tenant admin user id?

Also, is there any other better solution that can be used for accomplishing our scenario using graph APIs or any other APIs?