Hi FG
That is a decent find. With strong work over the next couple of years it should be well on it's way. With bonsai, trunk root development especially, it is all a matter of give and take. We want lots of roots and branches to speed the development but we also need to remove alot to direct the growth where we want. This means any work we do usually has a down side as well and concessions must be made as far as growth goes.
Since we can't do much on the roots at the moment I would be looking at setting the tree up for a drastic root prune next spring.
I would chop hard now or very soon. Just as the leaves have harden off would be an acceptable time. I think just above where you circled the shoot is a good idea for now. This will encourage a multitude of branches lower down. I would then hedge trim this for the rest of the season. let it grow out strong then hedge trim back maybe 2-4 times in the growing season depending on vigor.
Then next spring as the buds swell re-pot back into the same pot with major root reduction on the thickest roots and give the rest a good trim. Make sure the tap root (if any, but I think there is) and other downward growing roots are removed. The idea will be to create balance in the vigor of these radiating roots. Plant the tree deep back in the pot so fresh roots are not hindered.
The work you do this season on the trunk should mean that there will be no need to trim much at all of the branches at the time of re-potting.
This means we will have lots of new shoots on the trunk encouraging the new root system to grow vigorously.
Depending on vigor in this second year after re potting you may just let the tree grow to encourage great root growth or you may again hedge trim or be a little more drastic and start to shape the trunk by choosing a new leader once the tree is obviously growing strongly again.
Good luck.

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