Hi @Ross V,
Per my research, there is no such powershell cmdlet to reach your requirement currently. We are unable to manage Site Closure and Deletion Policies by powershell. You could check the list of the method in SharePoint Server
And I feel regretful to inform you that it turns out to be a by-design one. It is highly recommended that you can vote this ticket. Many features of our current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding.
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