Getting the third shot six months after the second seems to be the accepted best practice at the moment, though there is some research suggesting that the optimum benefit might be closer to 12 months.
But as with the initial shots, in the midst of an ongoing pandemic, the time to get it is when you can get it. Whether it's 6 months or 12 months or anywhere in between, the key is getting the boosted benefit.
As for waiting for revised versions of vaccines tailored to variants, that's still pie in the sky. Again, the best vaccine is the one you can get. And the way things are going, it's probably not going to be your last shot anyway.
As for waiting to "need" it in Hong Kong, everyone waiting for things to go south, then clamouring all at once for a booster doesn't seem like a good scenario from a public health perspective.
When community transmission gets re-established in Hong Kong (inevitably I would think), in light of the newer variants, it's liable to happen very quickly. And that's not the moment I would want to be scrambling for a shot.