I tried to make this long post, but then the forum gave errors and then it went offline.
Let's see if it posts:
You can see the pink one is already growing new leaves. Removing all flowers will slightly speed up the emergence of new shoots from the axil around the flower buds.
These azalea will not bud back from the trunk. If you prune a branch to old wood only, it will grow new shoots from the nearest dormant bud at the position where you cut it. And all dormant buds elsewhere will stay dormant. (satsuki are a bit different) To get reliable backbudding on your trunk, you need to cut away all leaves. If you do that, it is kind of like an equal playing field for all dormant buds anywhere. In spring, the tree has saved energy to push new growth. It is going to have to do that anyway because it loses most of it's leaves in autumn to prepare for winter. So when you prune in (early to mid) spring, you are not removing any invested energy because you are not removing shoots it has just produced but not received much energy back from. And then it has the whole growing season ahead to grow that new growth out. If you have a long growing season, you can do it in the middle. Then, you are removing some new growth just after it was created. But the plant has been growing and collecting energy for a few months now. So it is not a problem. If you prune the pink one as it is now, you would be removing these new shoots it has just created. If you prune the white one, you are still in time. But the pink one kinda is a bit late. That said, you can still do it. It will just be less ideal. If you have a large azalea nursery in Japan, you can't do everything at the exact ideal moment. Because then, everything has to be repotted at the same time. Everything has to be pruned at the same time, etc. So they can't adhere to the perfect timing method anyway. If you prune late summer, it will also push growth. But it won't have a lot of time to grow before the growning season ends. If you really prune early autumn, you will still get new growth, but it may not harden off properly for winter.
But what you don't want to do is prune your azalea a little two or three times a year for several years. And then cut it back to nothing. So cut away now what is going to undermine your future trunk line. Then let it grow freely. And when you are semi-satisfied with how fat the trunk is, cut it back to just the trunk. And then if you are truly happy and don't want to repeat that cycle, put it in a bonsai pot. Because azalea are plants, not trees, and because they have flowers, having a stick in a pot for an azalea is fine. But if you want something that looks like tree, it will take many years growing in full ground. And that growth should be as uninhibited as possible as long as that doesn't cause reverse taper. The thickness of the trunk is dictated by the amount of foliage surface area it has to support.
