Trim those long branches back. Larch are very terminal oriented and the back buds get suppressed by the terminal growth. Also, long sacrificial branches don't work on larch for trunk thickening. Instead treat sacrificial branches like regular branches. The extra foliage works better. Otherwise...
What you want to happen is have vigorous early growth to thicken new branching. Wire them down in late spring and fertilize heavily. Next year, prune it for structure and repeat the process. In a few years of this you with have strong tapered branching. After that, go for ramification.