Hmmm, the answer is tricky, but solution is quite simple.
Build NO offensive capships (or next to none).
Why?
They are big, slow, expensive and have to be crewed by the thousand.
Instead, concentrate on developing small, quick craft, that can be manufactured in massive quantities, on small instalations, not requiring so many resources.
Imagine, if throughout all those 20 years, instead of building and fitting the Colly the GTVA concentrated on fighters and bombers. They would be built in hundreds of thousands, able to overcrowd any Shivan fleet in a matter of minutes. 80 Saths? No problem, we send 800 bombers with an escort of 1600 fighters (two fighters per bomber) and they take care of SJ's one by one. Impossible? I don't think so.
Think about it, all that would have to be built would be carriers, and since fighters posess the ability to jump to subspace, they wouldn't even need to be brought close to the battle scene. They could be masked by the stealth technology. Take the mass of the Colly and distribute it among fighters and bombers. Imagine this thing dispersing ^^
Of course, Shivans could do the same, but then they wouldn't have the capability to destroy planets (Lucy) or stars (Sath). Sure, they could overwhelm us pretty much with fighters, but they would accomplish nothing. For the defensive army, which in fact the GTVA is, this course of action would prove the most profitable I think.
If there is a flaw in my way of thinking, I'd be glad if somebody pointed it out
