“The problem is, that preview image gets re-uploaded to Slack’s CDN, and is in high resolution and is now accessible to everyone in my Slack workspace.”
Paste a link to a private Google Doc in Slack and the Drive app renders the first page with your permissions. The preview is an 800 by 1035 image anyone in the workspace can open, even people with no access to the file. File Previews are on by default. Google’s sharing controls mean nothing once Slack makes its own copy.