Thanks for the excellent observations and advice. You are correct when you recognized that I am growing it in the oak style rather than a traditional bonsai shape.
Let this be a lesson for all
My vision of an oak is what I can see around me : we have several species here, and whether they are isolated trees or grow in forests, they are definitely more or less "moyogi".
The latin name of the tree is important too : a young tree growing in a particular climate might look very similar from a different subspecies that will have very different characteristics when an adult tree. Whether a Quercus robur, Q. suber, or Q. Ilex, very few mature trees have multiple or even double trunks, although it is frequent to find young trees or new shots from a cut buttress with several branches/trunks. After a few years, only one trunk remains.
It makes me think of Taxodium distichum : most of the bonsai you can see are conical shaped with a jin at the top; when you see pictures taken in the bayous or florida, they have flat tops!
So, bonsai aesthetics is cultural, environment-based (not sure at all this word is good English, but I think you know what I mean), and relative : you want to grow it in the oak style, well, this here looks like a traditional bonsai shape. Maybe using accent plants, and accessories in a display when the tree is ready would help to understand its inner nature better... Which perhaps means breaking off from Japanese 'canons'.
(My 2 €cents worth...)