You won't like this, but I have faced this problem. I do my work direct from Qt widgets, no QML/Qt Quick, so I don't know whether this is useful/applicable to you or not. I actually "pre-process" my QSS stylesheet file(s), before handing them off to QtWidgets.QApplication.instance().setStyleSheet(css) in code. So for example example my primary QSS file starts with:

/* Set the base font size for all widgets */
* {
    font-size: <FONT_SIZE>pt;
}

and my code, having read the file content into css variable, is like:

    # substitute all "<FONT_SIZE>" with the actual font size wanted
    fontSize = Settings.current().getDefaultFontSize()
    css = css.replace("<FONT_SIZE>", fontSize)
    biggerFontSize = Settings.current().getDefaultBiggerFontSize()
    css = css.replace("<BIGGER_FONT_SIZE>", biggerFontSize)
    ...
    QtWidgets.QApplication.instance().setStyleSheet(css)