It seems to me that the core problem for this and similar fleet combat issue, where planets build armadas faster than they are destroyed, is that damage is more or less linear in time, and since it has to be slow enough for the player to react, it can well be too slow to overcame breeding/construction.
The way I believe would work much better if there were phases, like preparation/skirmish phase, where forces gather but nothing much happens, and then a battle/invasion phase where everything dies in a few solar days. With a player visible counter until phase change even, to reduce confusion. And if the skirmishing phase has an actual effect, like creating force multipliers for the actual battle, it would give player something to do other than charging headlong into the blob.
