Repeat after me: Corporations and LLCs are not individuals. Corporations and LLCs are government creations. Corporations and LLCs do not have natural rights.
Corporations, just like a group of concerned mothers pooling money for a billboard, are a grouping of people. And shareholders have rights to spend their own pooled money on speech, just like the mothers have that same right.
This is disingenuous - a group of concerned mothers does not have a government-created liability shield, for starters.
In general, shareholders are always able to pool their money directly in the manner you are stating. This being uncommon shows that the situations are not equivalent.
I know that's part of the Supreme Council's current justification for why corpos deserve natural rights, but I don't really see what it actually has to do with anything. Regardless of how corpos might be regulated, the individuals associated with a corpo are all still capable of exercising their free speech rights in their own individual capacity. Declaring that the government-created legal entity should not be able to direct individuals to speak a certain way fundamentally preserves those individuals' rights. The corpo's owners are also still able to freely speak on the corpo's interests using their own individual rights.