Got this trident maple last Easter. Finally through a lot of fighting the tree has put on a lot of growth the last couple of months. So as the leaves start to drop most if not all the branches grew right were I wanted them. Now they just need to thicken.
I’m wondering while these branches or sub trunks thicken is it possible to let some of the secondary branches grow at the same time to help tree fill out by trimming them to keep them smaller?

Hello! Cool maple you have there...

About the branches, I think you can let secondary branches, but maybe you'll have to cut them back anyways once the primary thickens enough, because that helps you to build tapper on those branches.

Is your idea to make a broom style? If so, you could select this branches later on and position them. And next season let the ones you selected grow long, trident maples thickens quite fast!

I think it's too early to prune it to fill, a good deciduous Bonsai should show you a nice branch structure once leaves shed

Could you please share your long-term vision for this tree?
If it were mine, I would have taken a very different approach, so a bit hard to reflect on next steps without knowing your vision.