Ads are inauthentic speech someone is paid to express. If entity A pays entity B to say or display their words or content, it's an ad.

This is not an impossible problem. It doesn't need to be perfect, and we can iterate.

Or, instead of trying to decide which speech is good and which is bad, we could let anyone say anything they want, across any medium the cost of which they are willing to bear.

Just wait until the sky is turned Coca-Cola red. Fair game, right?

You seem to be describing some sort of anarcho capitalist fantasy land. There’s a reason no single country on Earth operates like that.