They shouldn't even bother building capital ships, I can think of four solutions that are far more effective.
Massive fleets made of nothing but bombers:
It's always bothered me how easily a capital ship can be taken down by bombers, but that's the way the ball bounces. Send 100 bombers at a sathanas from behind, 'nough said.
Unmanned missile warfare:
Take a meson bomb or something stronger, and put two separate engines on it. One engine is used to warp into a system, the other to make an intra-system jump. Send a dozen of them at a capital ship, and you've won the day.
Junk/Asteroid warfare:
Collect the debris of old capital ships, place a massive engine on the back, and ram away.
Dyson Sphere-ish defense:
Create a perfect sphere that can be placed over a jump node, anything warping in will be destroyed instantly.
------
As far as the whole "but the Shivans were doing
this" argument goes.... We have no knowledge of their society. Any attempt to understand such an unusual species is doomed to failure.
------
Think of it like 9/11. Why haven't the US built another World Trade Center at that spot? It would carry too many bad memories.
Firstly, construction in the United States has gone at a ridiculously slow pace since the '60s or so.
Secondly,
http://www.projectrebirth.org/about/index.html