Just some ideas.
1. This cost would not be in "damage" as it was before. Why? Abusable. If I send 10 ships, I lose one, but if I send 5+5, I lose none, I just have to repair them.
2. When you move the fleet from one wormhole to the other, you have to pay for it in credits. If you do not have the credits for it, you cannot move. I believe a 5% of total fleet cost would be reasonable.
These first two could avoid ninja jumps and quick sniping operations for big guilds.
3. Make the wormhole a level 1 jump gate. Or make it a "level 0" jump gate, just so you can send stellar units out of that galaxy.
This last one would allow any player to send the stellar units out of that galaxy.
4. Add an extra requirement for jump gates: level 1 orbital shipyard + level 1 orbital base.
The person that would build on the base would require more than the technology, having to spend more time building on the base instead of settling base + jump gate rush.
Or just remove wormholes. I was always against them anyway...
Getting Sandy to build fleet is harder than getting Panda's to reproduce in captivity.