On a dashboard, when no segment has been added on the url when the page is loaded, I want to set a segment by default which will be the first item of the menu.
Just to clarify, this means that the /dashboard page wouldn’t exist. Even if people typed in mydomain.com/dashboard, you wouldn’t want them to land on that page, you’d want to redirect them to /dashboard/firstItemOfTheMenu page. Is that right?
So for your first question, yes, you can do this. If the items in the navigation menu are always the same, then I would:
create one page for each item with the structure you mention
setup a redirection so that when someone tries to access the /portal page, they are redirected to /portal/repairs page
For your second question, if you use multi-page sections, the result on the published app will be similar to the one in your video. Inside the editor, we change page completely and reload everything but the published app is a single-page application so will only load new sections.