A multi-suggestion to potentially encourage stealth tactics and to allow non-deathball strategies to strive:
A special upgrade to sensors that are only available for larger passive designs, such that every 5 day tick, after normal detection, these upgraded passive sensors (thermal and EM) make a check against all sources of signatures (detectable or not) where:
Distance from contact < (Total signature strength of all contacts currently not seen using normal passive detection)*sensor strength * 1000km.
For every emission source that passes this check, the game takes the mean of all bearings of these sources, weighted based on emission strength, modified by range, and checks for all ships within an angle from this mean.
If there are ships within this bearing range, a check is made, going:
(Total sum of all signatures within angle)*sensor platform sensitivity *1000km...
Where was I going with this? Scrub it. Lemme start over.
A special upgrade for passive sensors that can only be applied to larger sensor systems.
Every 5 day cycle, the sensor makes a check against all contacts within the system with 10 billion kilometers (or just checks all if that's theoretically faster for game performance)
Contacts outside of normal detection range generate a "subsignature" on this cycle that isn't directly reported or view-able.
At this point, the game checks all ships that have a tight grouping of bearings from the sensing ship, and adds all their signatures together.
If any of the contributing ships ends up being within:
Sensor sensitivity * cumulative signature strength * 1000km
distance from the sensor platform, rather than generating a sensor contact, the game makes a special contact at the sensor platform's position on detection (viewable only by the owning race of that sensor platform), and it reports the average bearing used in the previous calculations, as well as the cumulative signature strength. Lost contacts would allow you to view old reports of this kind.
Anyway, on to the second idea:
-Make base ECM stronger
-To compensate, make it so that ecm is significantly mitigated by having multiple active sensors viewing the target. The effect is 0 when the sensors are at or near the same angle from the target, but increase if the ships doing the sensing are surrounding the target from different angles.