I think no matter what you do with the DoT, as long as it still angers all the ships on the planet you're going to end up in the same boat. No matter what it does, if it takes a few minutes to do it and the AI ships are coming to eat you while you're doing it, effectively it doesn't matter. You either have to warp out quickly, or kill those AI ships with your fleet. And if you're killing them with your fleet, then they're dead and the attrition effect has no importance. You've accomplished just what you would have done by sending in a fleet sans attritioner. Again we come back to the fact that it might well just be emitting an 'aggravation' pulse.
Solutions could be:
-Making it cloak while attritioning. That's pretty clearly overpowered without even needing further investigation.
-Making it extremely armored. That's interesting but you still end up with the same dynamic. If you make it so ridiculously armored that it can sit around and attrition down a planet while the entire population beats on it, it's way overpowered. If you don't, then once again you're bringing your fleet in to deal with the system, and we're back to square one where the attrition doesn't matter.
-Making the damage more focused. I.e., allowing it to execute one fat attrition pulse and then run for the hills. This semi-undermines the name of the unit, but not exactly the spirit of it. Is it possible to have it do percentage based damage? Like a pulse takes off X% from everything on the planet, after which it has to go recuperate for Y minutes before it can be used again? Something to consider anyway. Also still allows it to be used for aggravating everything on the planet if you so desire. This could probably be balanced with some number fiddling, although I'm not sure how you'd balance stacking a lot of them on one planet. If you made it X% of remaining HP though that might work. At least, until some smart guy puts ten of them on one world and has them all pulse at once. Maybe just flat damage numbers makes more sense. I'm just trying to think of a way that you couldn't use a huge force of these to cream every planet you come across.
-Giving it a repairable force field. Some sort of tiny bubble (not for use by other ships) that allows it to sit and attrition a planet for a while even under fire, but then forces you to run when it gets low. Probably hard to know exactly how strong to make it, but numbers fiddling is a problem for engineers, I'm an ideas guy.
-Making it really fast. Send it in and then play chase with the AI ships until you wear them all down. OK, that was a silly idea.