Without a marketplace addon to do this, I agree you will need to use pagination to get all the users with the REST API.
To watch for just the changes, you could set up a webhook to push the audit log events related to users, and then parse that in another tool. It will not have the group change information you note, but the created-event may be enough to help with your REST API calls. https://developer.atlassian.com/cloud/jira/platform/webhooks/
Kind regards,
Bill
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