In the end, a core framework integration with Etch is needed as I found Core to be more flexible and less opiniated than AutomaticCSS.
But in the meantime, styles are working quite well.
Main problem is that core framework declaration are affecting Etch interface while we will want to only affect the iframe preview of the Editor, not the Editor interface.
How could we declare something to left the editor untouched.
Etch have a way to enqueue for the preview. https://snippetnest.com/snippet/enqueue-custom-stylesheets-in-etch/
We will need a fix with a way to exclude Core framework from Etch admin Editor Interface but to continue to load it only on the Etch Iframe Preview window.