Give them a survivability ability that you can research, maybe something like "Evasive Maneuvers" where they have a 10% chance to shrug off each attack volley. That'll make them more like a persistent swarm of killer bees, rather than a swarm of gnats.
I don't think Corvettes should be anti-Titan. Just let the anti-module ships fire at Titans, with a chance for the titan to evade their shots (i.e. the shots would be far less effective against a Titan than an immobile Starbase, but still better than shooting it with conventional weapons). OTOH, to make Corvettes more useful, I'd be in favor of increasing the maximum amount that they can slow down jumps. This would allow a well-prepared player to field Corvettes against an enemy Titan to prevent it from escaping through the phase lanes when it's low on hull. You'd still need to be able to destroy it, probably using a combination of capships, heavy cruisers, starbases and your own Titan; but at least this way you'd be able to ensure that the thing actually dies, instead of escaping with 10% hull.
This is important because otherwise Titans don't exhibit the risk and the commitment that Sins is well-known for: it takes so long to destroy one that any average human (or a smart AI) would be able to survive all but the most catastrophic defeats with their Titan intact (especially if the Titan is defending a system owned by its own empire; if attacking an enemy system, Phase Disruptors may be a slight problem if you can't destroy them on your way out).
By allowing Corvettes to effectively block the escape route for several seconds longer, it makes the escape that much more difficult, giving the other empire a large window to destroy the Titan.
Even as someone who plans to use the Titan as a key component of my assault fleets, I would like this mechanic very much. Before Titans, the "commitment" of capital ships to a battle was a fairly serious move: you needed to scout enemy grav wells until you were really sure that you could take on the defenders, and make sure you either cut off reinforcements or ensure that the nearest significant reinforcements are far away. If you failed to do these things, you could easily warp your cap ships into a grav well with multiple things preventing them from leaving, and their survivability is such that they couldn't escape in time before being destroyed. Not really the case with a Titan; even if they have multiple phase disruptors, you can usually kill them and still get out.