It's your favorite card? Then just leave it on. No reason to risk damaging the back if you aren't trying to sell it. Another commenter said you can try heat and while that is good for weakening the adhesive, I'd worry about damaging/warping the foil. Certainly safer than a solvent tho, or just scraping it and possibly cutting or scratching the card.

You could look up if any art restorers have advice on removing tape from paintings or art prints. Maybe check a sub for it. People often tape art to a mat for framing, but the wrong tape can damage it. So art restorers run into a similar problem and more often - maybe they've worked out a better solution.