Ciao Roberto,
I have a different idea - without using extra hacks and custom CSS. Particularly as you have difficulty with this task.
Becuase on those subpages you use SPPB already, maybe instead of using Page Title from menu, just create/add a new Section that will be first on layout grid with all needed content and settings. Then you would have full flexibility what should be there and how should be displayed. Then Save that Section and reuse with different variants. Of course disable Page title from Menu item settings for those subpages. That's all. In my opinion, it is more simple for you to edit now or in the future days.
Here is example, yes, we used that method many times in our SPPB Layout Bundles.

and here is view from it
