Dynamic range will be limited by the converter's performance, anyway. It might go into some pulse skipping mode, for example. At that point, turn it off completely and use another method, say just a linear source since you probably aren't worried about efficiency down in the mA?
The resistor divider and DAC work as well as the components do; dynamic range means getting arbitrarily close to zero, which is just a calibration error for the DAC, and the resistors can similarly be calibrated out as a gain error.
Perhaps the converter's reference is drifty, so that calibration isn't feasible to meet some stability requirement? But then, if it can't hold zero, it won't hold any other value any better!
Note that op-amps cannot help here as their output goes to the same thing, it all comes back to the converter's error amp and ref.

Tim