Yes, if the states were to take over many of these things, obviously federal taxes would need to dramatically decrease (luckily, the vast majority of federal spending is doing the things that I think states should do anyways, so you'd be simultaneously dramatically decreasing federal taxes and federal spending).

You couldn't just have the states take over these responsibilities and have nothing else change. My suggestion is in fact a pretty radical change in how the US federal government works. I'm not under any illusion that this is likely to happen. The ratchet of power unfortunately only goes in one direction.