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Hi there I definitely admire your creativity on this one, but unfortunately the Google Tag Manager loads tags asynchronously.  That means that the tags are added for users who execute javascript on page loads, but it means that the copy of the page that is crawled does not contain the tags.  All of this means that the Google crawler won't see a noindex tag on the crawled version of its page if it's loaded via the Google Tag Manager. I think your reasoning for noindexing the pages themselves is a very good one.  You're removing pages with thin or potentially duplicate content from Google's indexing, which is healthy, while keeping the content on the page for the user, which again is a healthy thing to do.  I can definitely see the reasoning. Unfortunately, the only way I can see the tags being implemented is manually by the webmaster.  If the site runs wordpress, you can change the robots meta data very quickly in the Yoast SEO plugin.  If you get to the page in question and scroll to the Yoast plugin section, you'll be able to select the noindex tag from a drop-down menu, meaning it can take as little as 30 seconds. Hope this helps and good luck with the implementation.

Are you by any chance using relative urls for your navigation? I think this issue will be down to your navigation/internal link structure, but I would need to see your site to tell your for sure.