I am French, and I will incorporate in Delaware. Is the ESTA dangerous in this case, for example, if I go to SF in June?
That depends entirely on what you will be doing when in the U.S. While you can't work in the U.S. as a visitor, there are lots of activities that you can engage in as a founder while in the U.S. as a visitor. See one of my responses above regarding permissible activities as a visitor.
Could you please share your contact details so I can reach out to you for legal assistance if needed?