I believe I had the same issue and was told that it was not possible with Source Navigator. If you have sub-menus, then the main menu does not work, as you said. I can’t remember the reason for this, but there was a reason given. I then changed my main menu and added links to that main page that then went to sub-menu pages.
The Source menu doesn’t support having a clickable parent page. This is how most Nav bars in the wild work as it is the most intuitive approach
Rationale: Having a menu link that does something on hover (showing the sub menu) means it is not obvious that it is also a clickable parent page that would take you somewhere. And what about on mobile - what should the link do there? Open the sub menu or go to the parent page?
Alternative option: These days I think it is best to set the child pages to be hidden from navigation and link to them from the parent page. You could also add direct links to the child pages in the footer if desirable/useful.
Or there are third-party navigation stacks available that do allow for a clickable parent page.
IMO the purpose of a drop-down is to contain a group of related links. Normally this done because it would be too much information to put on one page, or because each sub header of the main subject is worth giving its own URL for SEO purposes. The parent is simply a title for this group of links and should be just that, not a link in itself.