...but the Imperial side will certainly need something to counter the CR90.

Isn't that what turbolasers are for?

I mean, surely small ships like corvettes are no match for Star Destroyers.

Sure, if the only factor in considering a ships usefulness is its firepower, but I would like to see you capture and hold 3 different strategic points on the same game area, at the same time, with a single Star Destroyer

Heh, just park some place in the centre and blow apart anything that enters them (perhaps sending a fighter squadron or something to occupy each one if required)

You assume that all of the target areas are in effective weapons range of the central point between them, and that the rebel force won't just blow your fighter squad to ash before moving in.