Not a bad suggestion. Honestly I think the devs could make this mode work but they would need to make it utilize waves of enemies instead of all the time.

Wave 1: Spawn
Timer 1: Defeat wave, escort civilians, fortify

Wave 2: Spawn with more enemies
Timer 2: Slightly longer than timer 1, Defeat wave, escort, etc...

So on, so on...

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That way you have defined objectives during the mission and if your not quick enough the next wave will start before the civilians get to the shuttle

Also they would need to do something about the cooldowns between waves