Hi @Leigh Hess ,
Thank you for posting in this community.
The easiest way to replicate an entire Modern SharePoint site to a test tenant is to use a three-party software.
But since you don't want to purchase third-party. Then you can only use the "Save as Template" feature: You can save the site as a template with content, and then use the template to create a new site in another test tenant. This method has some limitations, such as a file size limit of 50MB and a list view threshold limit of 5,000 items.
For more detail steps:https://answers.microsoft.com/en-us/msoffice/forum/all/cloning-a-sharepoint-site/b82f3e2b-7546-4e56-8df4-8ef566698529
Note: Duplicating a site does not copy workflows, alerts, or navigation settings.
It is currently still recommended to migrate SharePoint Online sites to another tenant using 3rd party tools.
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