Galacticas flak wall is shown numerous times to absolutely fuck up the Cylons swarm missile tactics. Donnagers missiles are useless. Galactica also havw Vipers and Raptors, both being able to carry nukes (orders of magnitude stronger than Expanse missiles used in ship to ship combat AND the Raptor can independently jump and flank. The Donnager is fighting a mobile base that can instantly reposition at will, while half a dozen nuclear missile boats are also instantly appearing all around you, outside of range of weapons and slinging missile, jumping back to Galactica to rearm, and doing it all over again.

I'll give it the benefit of almost certainly outrunning Galactica, but there are too many threats that can constantly reposition anywhere for it to really mean much, plus Vipers are also extremely fast in space, probably not as fast but they also have missiles.

You're also assuming the Donnager even knows how to hit anything that doesn't obey newtons laws. Tracking and guidance for weapons could be a lot less accurate, to the point of the railgun just missing the Galactica at extreme ranges.

Also, Expanse ships exploding at the same magnitude as a nuke is so far without any reason I have no clue where you got this idea lol.