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The answer appears to be that this is not possible:
https://docs.microsoft.com/en-us/answers/questions/581947/unable-to-update-scripteditor-webpart-csom-powersh.html?childToView=582966#comment-582966
Testing by a Microsoft employee respondent confirms my findings. This may be possible with the snap-in, though I am not running PowerShell on the SharePoint ...
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Try using below PnP PowerShell code. It works for normal document library. I was not able to test it for document library reaching threshold limit (I don't have enough documents on test environment).
Connect-PnPOnline -Url https://<tenant>.sharepoint.com/sites/<siteName> -UseWebLogin
Set-PnPField -List "Documents" -Identity "...
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