I think most people would call this tree a pre-bonsai stock.
First, this tree is not a sumo and probably could never become one. Sumo style bonsai are about as wide as they are tall.
So I would cut back and redefine the branching to be closer and denser to the trunk. Especially if you want a powerful image similar to a sumo. Right now it looks like an oak style but the ramification needs to start much closer to the trunk.
Hard to say where the best front is etc without seeing it in person but if your first pic is the front then maybe something like this. Broom style.
If you just want to keep as is and develop ramification then you would still need to cut back your top branding closer to trunk and develop taper and secondary branching close in.
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