Is there an optimum keyword density ratio for SEO copywriting?

Hmm. .... as a self proclaimed "SEO expert" you really should know the answer to that already. Perhaps "aspiring SEO expert" would be a better tagine.

In my view or experience, there is no ideal % or ratio of Keyword density, I prefer 1-3% of keyword density in a content.

It is around1 to1.,5% as of now. But if you are using appropriate jargons of your niche, it can be less. The importance should be given to the user experience and blending in closely associated words or concepts. And it will automatically help you generate more traffic over time.