Maybe is that I know the game too well XD. I know which enemies are going to spawn and where, so I can assault them and salvage all the enemy ships I can. You should build alway salvage corvettes and salvage everything you can. Some of my frigates were ion array frigates, though I retired them and bought iron frigates because their armor and damage was too low.
Also try to have max heavy corvettes and when you have multi gun, build multigun. Dont rely ever in fighters, its a huge loss of resources… Though my corvettes have been wiped a couple of times too.
But basically is that, salvage everything. I have right now I think that 7 destroyers, 3 missiles destroyers and 3 or 4 cruisers after the mission where you help bentusi. I know its tedious because you have less salvage corvettes than in HW1 and the salvage option doesnt work by drag… But still…
BTW in the next mission to asteroids you have to build max bombers and corvettes. The more you have the better. If you have not many of them the mission is going to be slow and could even wipe.