Some cultivars are branching types. If you feed it a little bit, you could encourage those buds to grow into nice new flowers. If you want new ones, cutting off the spent central flower head would help as well.

And you really grew this plant indoors? that`s quite amazing, I thought they would require more light.

O, and feeding, in a pot means liquid mineral fertilizer, whatever you can lay your hands on.

It has recently started to wilt, which I realize is part of its life cycle.

==>>> not that i am aware of ..

the flower head may have turned down .. but that isnt wilt .. it sunflower normal ...

also ... did you know... they follow the sun ... i dont recall the term ... but on a weekend.. observe it during the day ... and you should see such.. though that may be complicated by the indoor thing ...

based on the pic above.. i say new flower buds.. and there is nothing wrong with your plant... dont go loving it to death.. because you think its wilting... and with its lush growth.. i dont even think it needs any fert.. other than what you are already doing.. somewhat to perfection ... [except for indoors ...]

ken