What was the point of any of Balon Greyjoy’s plans?

Yes, Deepwood Motte gets you a stronghold with access to the sea, but you’re so far away from the rest of the action that you can’t really support any of the other Ironborn forces, so you make yourself somewhat of an irrelevance. And honestly, Bear Island, being an island, makes for a much better choice for a stronghold in that region.

IMHO, the reason Asha takes Deepwood Motte is to give Bran and Co. a reason why they can’t take shelter with the loyal Glovers and have to keep going straight north to the Wall.