so using signals looks unnecessary
Then don't use any! There's no one forcing you to.
I heard that the correct way to use signals is to "signal up and call down"
That's at best a rule of thumb, not the "correct way" to do things. Actually, there is no such thing as a "correct way": you can achieve exactly the same things with both and as long as it works for you, there really isn't any reason to change something.
Thing is: If a node calls another nodes method (doesn't matter if that node is a child or a parent node) and that node doesn't exist yet/anymore/in the new context, that will lead to an error. If, on the other hand, a node merely emits a signal this will always go through as long as the node defines that signal. And it's the other nodes responsibility to connect to that signal. If you remove a connected node, nothing will break.
So under the assumption that you might end up re-using a scene in a different context (with different parent- or child-nodes) using a signal to split up responsibilities makes sense. If you're sure you won't end up in such a situation or don't mind changing a few method calls, you can achieve the same stuff with direct method calls though.