Hi,
I don't have much experience with exporting for android, but I would try:
1. As startup scene set some relatively simple one as only texture rect or something like that.
2. In that startup scene load real first scene in the bg.
3. When finished loading add loaded scene before current one.
4. queue_free()
current scene.
By "before current one" in step (3) I mean that root children should be like:
[some autoloads here]
-RealFirstScene
-DummySplashScene