If they're sure they can invade whenever they want, does it make sense to do it right now? What would they gain by sending all the good tech and researchers straight to the US? The longer they wait, the more advanced the tech they eventually steal might be.
It's the golden goose story, but with a strategic rationale of when to kill the goose.
It's not all about TSMC. The CCP has been very vocal about wanting to conquer Taiwan since long before TSMC even existed.
Also, the longer they wait, the more tech gets transferred to the US, so even if it were all about TSMC, they would still be incentivized to act as soon as possible.