The things you desire are sort of at odds with each other. The way to encourage ramification is by judicious pruning, while the road to maximizing trunk caliper includes allowing the plant to grow unencumbered by pruning. Still, if you want to have a hand in establishing the basic shape of the tree, you'll need to be proactive and do some planning.

Here is what I would do: I would consider only the first (from the bottom) pair of opposing branches as parts of the composition. Let the branch on the left and the main stem grow unencumbered for a full year.

The branching pattern for P afra is alternate/opposite. This means that the branches are arranged in opposite pairs ant they alternate (every other branch) between vertical and lateral. That is, on a horizontal branch, a pair of secondary branches that are emerging from the top/bottom, will be followed by another pair that emerges left and right. So - after the first pair of branches that occurs on the branch you're keeping (on the right) emerges growing laterally (left/right instead of up/down), I would tip prune the branch, which will cause 2 new branches to emerge from axils (crotches) of the leaves you retained. Essentially, you'll be beginning ramification of the first branch as the top of the tree grows.

If you simply want a broom-shaped tree:
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you can allow the trunk to grow as fat as you want it to be, making sure it stays straight or has a gentle curve.

Al