Save up the credits until you CAN field a mixed, balanced fleet, because when it comes to BACs and K-Wings, there is no magic bullet that will stop them because they are powerful, well-rounded units. Refuse to fight for space control and hold the planet surface, and the AI will rarely push beyond that one planet - usually after the first ground invasion most but not all of their fleet will leave orbit, and you can pounce on the remainder. But the reality is that defending in space is FAR more expensive than defending on the ground, credits-wise, and since the AI gets free money and free ships, you can't expect the space game to be fair play.
In my experience as a mostly NR player, Lancers experience a drop in their danger rating post-era 3 - NR fighters are better than they used to be, NR bombers are WAY better than they used to be, and NR frigates and cap ships are better ship-killers, meaning it's less of an issue for them to spare a moment to blast apart your Lancers than it was when the MC80bs were simply buffing power to shields and waiting for the fighters to sweep in and save the day. Even without the big ships at play, a late era NR fighter/bomber wave can smash straight through a line of Lancers taking minimal losses, which is why you need a lot of fighters of your own to distract them and give the Lancers time to work. The Lancers are still important, because without them backing up your fighters, your fighters will lose in the end. And the cap ships are important for protecting the Lancers and carriers once the BACs get close... see how it all ties together? The IR fleet is a machine; it needs all its parts to work, but when it does it is a thing of beauty.
EDIT: Wait a second... you have a Sov?! Drop that bad boy in with Lancers and Venators (or is it MTCs for them there?) backing you up and slaughter at will - just keep an eye on how much damage you're taking and run in time if you have to, hit and fade their fleet until it's dust. Drop a probe droid on the mists every couple weeks to see if their fleet buildup is getting too big, and before it does drop in, slaughter everything, and leave.