I don’t think it’s possible to migrate all contents in multiple site collections from one tenant to another tenant by using PowerShell.
You could use save site as a template to achieve this. Here’re steps:
(1)Go to SharePoint Designer, open the site, click Site Options in the ribbon.
(2)Locate the property SaveSiteAsTemplateEnabled and modify the value to True.
(3)Go to "http://yourdomain.sharepoint.com/sites/site collection name/_layouts/savetmpl.aspx".
(4)Click include contents.
(5)Go to solutions under web designer galleries, click the site template, save as file in the desktop.
(6)Create a new site collection in the central administration, in the template section, choose "Select template later".
(7)Go to the new site collection, click solution gallery, upload solution.
(8)Go to site settings -> Site collection features -> Activate the site template.
(9)Go to the new site collection, in the custom tab, choose site template.
Or you could use ShareGate.
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