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The page being indexed already is not a factor. You want to add the tag to the primary page and all variant pages with parameters. The next time those pages are crawled, Google will see the canonical tag and adjust the URL accordingly. Your question is difficult because the answer would depend on how your CMS is coded. Generally there is a template which is used to create the pages. Your template would need to be modified using a conditional statement to provide the canonical tag for URLs with parameters. This would be your best solution.

Since GoogleBots can't access the premium regardless, is it redundant to have on the robots.txt file? You are indeed correct the best practice is to minimize your robots.txt file. That is why the very first sentence in my initial response was the suggestion to NOT include your premium sections in the robots.txt file Will google and other bots will hurt you if you have premium sections that they try to crawl but can't? No.

This is exactly  what I'm trying to determine as well.   My initial findings tilt toward very low seo value but haven't found a definitive answer.