Those knuckled knees came about by pruning off at the same point year after year (you see the same effect on those trees (robinia?) which are pruned to create a ball on a stick effect.

Pruning is a funny thing. With a few exceptions, it actually promotes growth and the harder one prunes, the more growth one can expect.

I would be inclined to prune your tree in the manner of a fruit tree - cut right back to within a few cms of the Y in autumn. This should promote branching of the two arms which you can then train from there by removing growth which goes in the wrong direction. You can rub off unwanted growth in the soft green stage, which doesn't leave any trace of pruning at all.

In subsequent pruning (which may be necessary to thicken up growth) lightly prune any more vigourous branches, and prune harder those which you want to promote.

Two or three years of pruning should be enough to develop a sturdy and balanced framework, if any further shaping is required. Once the framework has developed to your taste, trimming off the spent flower heads should be enough to promote bushy growth.