The big corporations already command a wide variety of industries. And different kinds of industries coordinate with each other, even if they are different corporation. Think just-in-time delivery of raw materials to manufacture a final product.
We also do have access to virtually unlimited amounts of data. Sure, some of it is not exactly useful, but much of it is. And we also have the technology to harness it.
A planned economy also wouldn't have to be more efficient in the same way. The point wouldn't be to achieve infinite growth, but to reach societal goals. Build X amount of housing. Make sure that enough food is available everywhere. Coordinate relevant industries in the fight against cancer. For most things, you wouldn't need to coordinate the entirety of the economy, at least not directly. Just the relevant parts in the relevant region. And if conditions change, you adapt the plan. A good plan is flexible.
A free market is not really different from a plan, in that sense. The two problems I see with free markets are that the aim is always, to some degree, growth and profit, and the competition. Having a choice, at least for consumer goods, is great. Not everyone likes the same apples or clothes. And the USSR had some bad experience with entirely removing branding, and therewith accountability, from things like bread. But with the need to outcompete each other, the alternatives waste so many resources on branding and marketing rather than making their products better. Those resources could be employed much more productively.
Yeah, I agree. All the power to the workers. My idea of a planned economy is probably not too far off from what you describe in that last paragraph. But I'm no economist, so please don't ask me to put forward a coherent policy proposal. xD