the 9cm pot is atiny pot. it needs to go into a minimum of a 1ltr pot. they are slowish growing but i suspect a couple of years before it would need poting on. daphne's are usually slower growing so may take longer.

S. hookeriana var. digyna is the one i have in dry shade and hasn't put on that much growth (in the ground) in the three years i've had it. it seems to do flower well and has a lovely scent. I'm not sure the humilis has a similar habit and whether it'll be worth putting it in a bigger pot.

I think the eventual spread is because of suckers. Whether you can keep it in a pot indefinitely depends largely on how big your pots are - this should not be a problem with this variety. Plants should not be put in pots that are a lot bigger than they need at present, but potted on as necessary.

The daphne really ought to be in the ground, in good, loamy, free draining soil, with a fair bit of sun (that's the hot stuff we used to see in the sky). In a pot the fluctuations in water availability, which are almost inevitable despite our best intentions, put stress on the plant and will hurry its demise.