Tax campaign related spending at 200% after you hit a limit, and distribute the money to smaller candidates so donating more becomes self defeating.

There is no hard cap and no limit on doing things that don't cost money so no one is ever prevented from speaking (that would open censorship challenges). It doesn't need to survive strict scrutiny because all candidates are treated equally making it viewpoint neutral topic discrimination. And Congress has a long established power to tax basically anything they want.