Maybe it's cheaper to build train tracks throughout Eurasia and evade rather than fight the US Navy.
If the US pirates Chinese ships, stationing light military forces on their trade ships directly should be cheaper than naval escorts.
The most efficient way to counter blockades in maritime choke points is to build alternative trade routes. Especially for all those canals. If I'm not mistaken China already has a project in Nicaragua that would bypass the Panama canal.
If you buy a fleet of glorious battleships, that investment depreciates in 30-40 years with new technology. But the additional trade canal will generate benefits that last at least 200 years. Buying fancy weapons is about getting better at competing with in the current environment. Building new trade canals is changing the environment to your advantage. That's on a higher strategical level.
Maybe China still needs some naval power, but a lot of US naval power can be negated by civilian infrastructure, that has a higher return on investment and further improves China's image towards being a benevolent power. You know the slogan "China builds, USA bombs"
If you engage in any kind of warfare, the most important factor is that you can decide when and where the battle takes place. In your example The US decides about those things by attacking Chinese trade. If China does reactive defense as you suggest, they would let the US have the advantage of dictating the terms of engagement . The more correct strategy for China is to not directly defend their trade routes but instead focus on finding easy US targets they can threaten to deter the US from hostile actions. That way the potential battles would happen on Chinese terms.
You think too small.