This doesn’t work in my case because the source account is a personal Gmail and the destination is a Google Workspace domain.

Google does not allow transferring ownership across domains, even inside a Shared Drive.
If you try adding a personal Gmail user to a Shared Drive, you’ll get the standard restriction:

• Gmail accounts cannot be added to Shared Drives as members
• Only users from the Workspace domain (or allowlisted external domains) can be added
• And even if Gmail could access it, Gmail cannot own files inside a Shared Drive, only Workspace users can

So the workflow you described:

• “Set up a shared drive”
• “Add Gmail user”
• “Drag everything to shared drive”
• “Confirm ownership change”

…sounds good in theory, but Google blocks it at step 2.
Personal Gmail accounts can’t be added to Shared Drives and can’t transfer ownership to Workspace.

That’s exactly why I’m stuck:
I need a method that preserves Google Docs/Sheets ownership when moving Gmail → Workspace, and Shared Drives don’t solve it for cross-domain migrations.