Try this Fix Checkboxes action. It is a single script step like this.
draft.content = draft.content.replace(/☐/g,"[ ]");
draft.update():
The first line says to update the contents of the draft by globally replacing all occurrences of the square character with the Drafts checkbox markers. The second line indicates changes are to be applied.
You may need to amend the action and replace the square character in the first line. I copied this directly from your post, but there is a chance it might be slightly different to what you have in your draft. Just give it a try and replace if required.
Hope that helps.