Okay
You either go total stealth, or partial stealth, giving them the same radar signature as a common missile.
Bury a dozen or so in a missile barrage attack.
Determine the number of missiles that hit their target by dodge, ECM and turret fire only to determine the percent chance that one of the drones got through.
One drone hit and game over.
You BGG drones are going to be much, much bigger than a missile. The smallest black globe is ten tons, so guesstimating would put your Drone at about 30 tons in size, particularly if you want to be going at thrust 8. It's going to be at least 40 tons if you want to be faster.
With a clever drone command module, those engines, the power plant, and the globe generator, you're looking at about 134Mcr of drone.
Let's say you launch ten of them at my 200,000 ton ship amongst a barrage of much smaller torpedoes. Now, here are your problems:
-The drones are bigger targets
-The drones can be targeted by main guns, not just point defence.
OK, so you said you will stealth them to radar...I still have Lidar, a densitometer, an EM scanner. I can see you, at least if I have a sensor ops section who is on the ball...
Well, anyway, lets say one of them gets through, and cuts my battleship up. Now, you have used
at least ten of your drones, which cost you...
1.34 billion credits.
First: I do not see you doing more than crippling a ship which is over 100,000 tons. You cut of a section, bulkheads slam down, iris' close, and those sections attached to the engineering section will still be active. You lose a section, but most of the ship is still functional.
Second: For that cost, and even for that tonnage, I can launch one hell of a lot torpedoes at you.
Third: I think you are going to need a lot more than ten drones to get through my ships PD and Main battery fire. I think it would be more like 50 to 100 drones. That would cost you more like 70 billion credits.