No additional tax is needed, instead just do fork them once they reached certain level. So, basically split each company once it overgrowth certain level of its revenue. This will allow more automation split more equally among more people. Basically, strong AI is a "one-man company" dream. Just do whatever needed to allow equal access to AI by every member of your society, and make founding new businesses quick and easy.

How do you tax companies once they've become more powerful than the government (which can be equated to a company representing humans), and don't need humans anymore because they've automated labor?

For now we can tax them because they depend on us. Once they don't need us anymore, it's over, they'll let us starve and carry on.