30 ship fleets is what i call a backup system defense fleet. For catching small enemy fleets.
In the endgame I usually have some 100-200 spare DB's lying around mothballed. I have 2 100+ fleets. So when a war is coming my way, i can bump the fleet up to 200 with the mothballed onces and still have some smaller fleets in the backsystems.
Once i have installed system shields, i can bring those smaller fleets to the front also. Anway, then the real fun is going to start: in my recent game against TDM AI, i have 2 ringworlds, 2 almost finised and another 4 will be finished within 8 turns. Yummy.
I cut down my fleet size down by mothballing ships
so the AI doesn't immediately attack me. That would slow things down because of the heavy cost of constructing 6 ringworlds at the same time. If they would do it now, i'm ready. Only 1 system has an open waypoint to another system of the AI (so i can jump in and toy with their planets). I already began concentrating forces there since the AI will probably start an attack soon as i am completing the ringworlds.
On of my gripes with SEIV is that while you are at peace with a race, they send in smaller fleets (+-8 to 10 ships) into your space and once they are at war, these little fleets can cause problems so you always have to keep some smaller fleets around yourself. I use the smaller fleets and large WP for that. Not sure what's best for this kind of defense. Maybe fighters are better, i don't know. I do know that building WP on all your planets ( what i did ) is very costly.
Anyway, that's why i like war, you always can keep your systems clean from those little enemy fleets and once you have system shields, nobody can come in through the back door so you know when and where they are going to come
Makes for a "cleaner" less chaotic game