Frigate and campaign do not mix. In normal online challenges it doesn’t matter rather you win 100% or 20%, a win is a win, so frigate rush has a place. If you ask frigate rush to actually win with a high percentage, well that’s just not happening.
The problem with Campaign mode is that it isn’t finished. You have fleets for “barbarian” planets, but there is no agency out there suffering through the same limitations as yourself. This makes the entire mode unsatisfying and a difficult sell when trying to get friends into the game. Galactic Conquest is wasted potential. I think it could have been the breakthrough expansion of GSB.
The campaign seems to be DOA. Obviously. It’s still in beta while Cliffski was working on a GSB expansion and there still seem to be problems. There’s been no input in the campaign forum since May. I haven’t played in a long time (never did play a lot) waiting for it to be finished but I think I’ll reinstall the base game, play that, and forget about the campaign as a waste of money.
I’ve been enjoying the campaign.
After I unlocked everything with honor points I jumped right into campaigns. It tought me to make fleets with at least .2 speed and survivability. (Which actually made my challenges harder!)
I’d eventually claw my way across the board and then BAM! hit a fleet that just owns me. I figure that’s just part of the game. What I find really annoying is when I scout a fleet (using the single fighter approach) then save the game, it seems like the next time I load the game there’s a different enemy fleet there. Very annoying.
Anyway, overall I think the campaigns are pretty balanced.
My only trick is that I don’t build anything right away - I wait until turn 9 or so (just before threat level gets to 100%) In that way, I have more money and crew to build with initially.
I admit my campaign experiences aren’t as current as yours, but still – 9 ??? Hell, I routinely waited until at least turn 70 before sending out my major expeditionary fleets. Sometimes I wait even longer. With that amount of cash saved up, I can actually build fleets that are worth my effort and (usually) won’t get mauled by the very first enemy fleet which sees them.
Sending out lone fighters as scouts is a different matter of course; I do that much earlier than turn 70. 
I share your frustration with the enemy fleet mix changing by the time your second force arrives. Formerly, the campaign didn’t do that. 
I found campaign rather quick to finish on the lower difficulties and i started aggressive from word go… you need to be willing to back up and lose worlds to push back out sometimes and i find holding a strong fleet behind a choke with 1-2 ships at te choke so i see what they got when they are coming for me till i’m ready to push them out works best for me.
Yeah, losing a world (escpecially if it’s only a turn until a retaliation attack) is no big deal. Since you don’t build infrastructure that gets destroyed by war that you have to rebuild, all you’re usually out is a little income, be it money or pilots.