I don’t believe there are any constitutional restrictions that would get in the way of the war, treaty making, or expansion that would be required of a federal

I don’t believe there are any constitutional restrictions that would get in the way of the war, treaty making, or expansion that would be required of a federal government aiming to annex foreign countries whose tax regime it didn’t like. Definitely some international laws that would violate, but you can choose how seriously you want to take those. There probably isn’t an authority that could force the US to comply with them at least.