"essentially speaking" is correct and usable in written English. You can use it to indicate that what you are about to say is a summary of something, or a simplified version of a fact. For example: "Essentially speaking, the government plans to increase taxes to cover the cost of the new healthcare program.".
Essentially speaking, inward migrants are more likely to be of working age than the population in general.
Griffith's language is so fluent, so confident, that it's hard to believe that he was essentially speaking it for the first time.
And so while novelists and attorneys both rely on the power and persuasion of words, they are essentially speaking different languages.
Essentially speaking, the editing work is to replace each curve of the sequence by another curve.
In each instance, Rodriguez essentially spoke out of harm's way.
"As I understand, they have effectively no market share – so they essentially spoke against freedom of speech and human rights simply in order to contradict Google".
(Because some of the debenture deals left open the option of using either cash or stock, some of Liberty's Sprint PCS shares were essentially spoken for in both sorts of transactions, leading to some overlap in the calculations).
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union