Writing a short paper pointing to a mistake in a 2012 journal paper that got significant citations, while discussion with the author not successful
If you are sure of your work, there is little reason not to write the paper and submit it. Cite the other work properly, of course, and make your case clear. If the editor of a journal doesn't reject it out of hand it will be passed to a couple of referees who will give their judgement on the work.
But you don't jeopardize your career by doing this. Your paper isn't public until it is published. But asking the editor first is probably a  mistake, since you can't develop your argument without writing the paper.
And, it isn't impossible that in writing it up formally you will find an error in your own work. Make the case. Submit it for publication if it is strong enough.