I tried creating a vertical layout with title widget and toolbar, and added that layout above central widget.

That's close, but you need one extra step.
Create a plain QWidget, set a vertical layout on it, add your widget and toolbar to it and then use setMenuWidget() on your main window.

EDIT: Or you can even make it simpler - just use your title widget directly as the menu bar.

I tried creating a vertical layout with title widget and toolbar, and added that layout above central widget.

That's close, but you need one extra step.
Create a plain QWidget, set a vertical layout on it, add your widget and toolbar to it and then use setMenuWidget() on your main window.

EDIT: Or you can even make it simpler - just use your title widget directly as the menu bar.

A QMainWindow has its layout as per https://doc.qt.io/qt-5/qmainwindow.html#details picture, and I don't see it offering you to change that framework. So using a QMainWindow feels like a square peg & a round hole. Are you tied to QMainWindow. If it's your own code and not tied, I'd have a QWidget with the furniture bits added to it. Like I say, until someone says yours can be done with a one-liner on QMainWindow.

Another possibility is to try inherited QWidget::setLayout() on a QMainWindow. Find out how they've laid it out, and insert your extra title widget before the toolbar ( how did you do your "added that layout above central widget"?). But don't blame me if it doesn't work or makes something go wrong!