By letting the branches grow out for a season, you add more bulk to the trunk of the plant, and you stimulate the recovery from the blunt trimming done before; In other words: THe plant puts more 'scare tissue' over the cut, making the closing down process somewaht faster. Also, bu trimming the branches back later, you add some taper to the new branches.
In principle, when you bring the branches back to the first or second node, you get smaller internodel distances because the first sets of leaves simply are close to eachoter. Check any plant you have trimmed, and 95% of the time you will find the first few leaves to be close to the stem, and not far apart.
At the current state the Hydrangea could very easily be wired & shaped; The reason for letting it grow now would be to crete thicker trunc and help it heal the wounds.
Personally, I would now already decide which branches are not going to urvie the next trimmeng and remove them, to avoid scarring; The side-branches are still young, and the wound created now will close very easily. After the branches harden off, the cut will take longer to heal, i.m.h.o.