If you just want to operate a DAC from low supply, use a 5 or 3.3v DAC. The special feature of AD5412 is the built in high voltage output for standard process signals (0-10V, 0-20 mA). So you'll use it when you want this kind of interface and then you need a high voltage supply.
A straightforward way would be to use 24V DC supply, as most automation interfaces do.
Stepping down from 24 VAC to 5V and up to12 V again sounds a bit strange. You'll also increase the power dissipation and generated noise of the step-down converter. But it's possible though, if you have reason to do so, go for it.