How to deactivate the welcome screen in LibreOffice for Mac?
There is no known setting to disable this dialog.

The "welcome screen" serves as a dispatcher to the different parts of LibreOffice (such as the word processor or spreadsheet). Because these applications cannot be accessed directly (they lack stand-alone binaries), the welcome screen is in fact quite functional when not launching LibreOffice from an existing document. Disabling it would not make much sense from a useability point a view, therefore I am not really surprised there is no setting to change this behavior.