Mind you, I’m 100% okay with using penultimate to mean second best,

That was my intended use of the word, but I'm not ashamed to admit that I fully misread the little info blurb in my search engine, which led me to believe I was using it appropriately 😅

I thought it said: a formal or literary way of saying it is not a superlative or beyond ultimate, as many people think.

...I didn't notice the period after 'it' (or the double 'it'). But the sentence right after that one is really the pièce de résistance! 🤌

But I very much appreciate your charitable view on my fumble.