Hmmm. I dont do this, but one option may be an apple airport extreme. Or something similiar with a hard drive. Or, continue to do what youre doing, but have youre computer back up nightly to an external?
You can make an alias of iTunes on an external drive connected to your Macbook and the iPads will then sync to that, I have done this with my iTunes for my music because I have around 20,000 tracks.