And every single one of those hulls gets a generic bonus applying to all drone types, EXCEPT the Ishtar. It is also notable that only the VNI & Ishtar have what could be called a 'projection' bonus, though the VNI just gets drones to you faster. Sentries on sub-battleship hulls are fine. Sentries with bonused optimal range and tracking on those hull sizes are not balanced, and you won't find that on any of the drone boats you mentioned save this one.

Vexor: +50% damage & HP
Myrmidon: +50% damage & HP
Eos: +50% Damage & HP
Tristan: +50% HP & Tracking speed (Only has bandwidth for a flight of lights, two mediums, or one heavy)
VNI: +50% Drone HP/damage, +25% Max Velocity & Tracking

Ishtar: +25% Sentry HP & Damage, +50% HP/Damage other drones, +37.5% Heavy Drone max velocity & tracking
+25% Sentry drone optimal range & tracking speed, +25km to drone operation range.

Only one of these drone ships has a bonus to damage projection (which is important for sentries) and gets a bonus to drone control range, requiring fewer fitting compromises in order to kill a target 100km away. No other ship listed here can sit at such a range from their target while making as few fitting compromises as the Ishtar to get their drones working so far away, nor has the drone bandwidth / bay to deploy three full flights of sentries.

The Ishtar is uniquely able amongst sub-battleship hulls to deploy sentry drones which are effective at long engagement ranges, control them from such a range, and carry enough drones to be effective over a sustained engagement. These things are why the Ishtar is broken. Remove the sentry application bonuses, keep the sentry DPS buff at +25%, and you'll find a much more balanced hull.