If it were my tree. I would think of it as a broom style of the type you'd see in a forest. Informal broom, using the branches already present. You have 20 cm of bare trunk, there is no changing that. I would plan on creating a broom that would be around 60 cm tall or even a little taller. I'd cut off everything above about 40 cm right now. The branches in the 20cm to 40 cm range will be your main branches and main secondary branches. You will re-grow the top 20 cm of the tree, this part will be all tertiary and higher branching, the fine twiggy stuff. So cut off the top portion that you won't be using, then sort through the remaining branches and wire them to arrange them in gentle partial arches away from the trunk. I can't see exactly what you have in there, but the final goal will be a tree with a silhouette like a tall wine glass. Plenty of space underneath for the understory trees to grow. Your understory trees should probably be somewhere between 20 and 40 cm, leaving this one as the main focal point.
And as
@sorce suggested you can use the cuttings to create the smaller trees to accompany the forest giant.
Your trunk is only 3.5 cm, forest trees are usually 10:1 or so as far as height to trunk diameter goes in bonsai shows. In reality it is more like 100:1 in an actual forest tree, but isn't relevant, we are making an image in miniature, 10:1 is a good ratio to work toward. You will get some thickening over the years, especially if you keep the majority of branches in creating the broom, but thickening won't be quick. A 60 cm tree with a 3.5 cm trunk would be roughly 17:1, which is not wildly off the ideal of 10:1, so I think it would be a good plan.
At least this is my first thought working from the photo you posted. It is a plan that keeps the bulk of the branches and foliage. By keeping a fair amount of foliage, it will be possible to get growth and thickening of your trunk.
If you go with the broom, as you choose the primary branches, and secondary branches, remember to choose a thinner branch in favor of a branch that is unusually thick, at each level in the tree you want the branches to be similar in diameter. The diameter of the branches should step down, become smaller at each level as you go higher in the tree. So try to avoid the ''odd thick branch'' shooting up into the next level.
I would maybe draw a few plans, using only branches you actually have, and kick around ideas over the winter, do the actual styling in late winter or early spring. Take your time, this material does have enough branching to give you a number of options. And consider Smoke's suggestions