So deciding what is and isn't banned becomes an inherently political statement.
You can exclude ads that mention or are paid for by any PAC, candidate for public office, or are advocating for or against any particular public policy decision, or mention the topic of elections or voting.
This would eliminate the majority of them, leaving bad actors that don't care about Google's requirements anyways. Another option would be to simply make certain keywords unpurchaseable.
Recognizing the truth or falsehood of a statement or series of statements can be complicated, but we're doomed as a species if an objective definition of truth stops us in our tracks.
Besides, we have precedent here. Have a look at their policies surrounding what's allowed in health ads[1], they're pretty extensive, objective, and granular.
[1]: https://support.google.com/adspolicy/answer/176031?hl=en
They already do that with their own employees. They censor and control what statements they print to control for accuracy. Journalism concerns in part determining what true statements we should print. I think the Economist and CBO do a good job of separating the facts from the fiction. I would trust them to fact-check statements. I would not trust CNN or Facebook or the Democrats to fact-check statements.
At the same time, if we're talking about advertising, in the US we can just rely on what constitutes political advertising under the law.
I would imagine that there would then be an incentive for potential donors to, rather then giving cash, pay for messages that fly under the radar but are designed to support a particular campaign.