Once you truncate the main stem, it can't extend and it won't grow any more leaves. What it WILL do is grow new branches from the axils (crotches) of some of the existing leaves. The greatest certainty of that happening is from the axils of leaves and possibly from immediately above any leaf (bundle) scars immediately below your pruning cut/pinch. You can train any new branch that occurs to grow vertically so it takes the place of the one you removed (becomes the new leader).

If you really want your plant to have a sturdier trunk, you should do more than pinch it. A 1/2" trunk that is 30" long won't bend nearly as much as a 1/2" trunk that's 60" long, so shortening it by half is actually what I'd do. If you don't want to shorten it that much, you should move right into a pinching program so as to avoid any branches becoming too long and exerting a lot of leverage that tends to make the trunk want to lay over. Allow any given branch to grow to 3 or 4 mature leaves, then prune it back to 2 leaves. You can practice pinching at any time of the year, as long as the tree is healthy. Habitually pinching all extending branches back to 2 leaves maximizes ramification (# of leaves and branches) and makes your tree as full as it can be. Also, the rate at which branches and trunks thicken is directly proportional to the amount of leaf (photosynthesizing) surface on the tree, so regular pinching helps (a lot) to keep trunks strong enough to support the foliage.

Root congestion limits growth, and particularly extension of the main stem and branches. It also causes the plant to lose it's older leaves. In severe cases of root congestion, the plant might only have 2 or 3 leaves way out at the growing tips of branches, and the plant becomes very reluctant to back-bud. Don't wait to repot until the plant is in severe decline. If the root/soil mass can be lifted from the pot intact, your plant will benefit from repotting. Potting up doesn't relieve the limitations imposed by root congestion, though it can allow the plant to briefly grow a little closer to its genetic potential. Still, potting up is better than confining it to a pot too small.

Al