Continuation of this post.
Basically, I emailed a publisher and two profs, asking for help on a paper I had written but not submitted. Turns out the method was not as novel as I thought, as an MSE post containing pretty much the same derivation was posted in 2016. Now, I'm trying to figure out how to break the news to them.
Off the top of my head, I considered saying "actually found it's been done before", but not sure how good that sounds. Probably this is as bad as I think it sounds, but any advice making it sound the best?