My Xenonauts 2 strat is the exact same as mine in 1, charge forth, for I can always easily replace the casualties. I never even knew the Aliens would disappear once the time runs out as my rush strat would always grab 10/10 by the time the turn limit hit. So I believe that the issue is the lacking strategies. I also do believe that changing missions to kill everything is also a mistake, perhaps the addition of differing types of missions? To add more variations of 'kill all xenos' and add a new type of mission like 'survive for X turns'
Like a defend VIP like a president (linked to increasing funding/decreasing panic or vice versa, get control of bodyguards and the president defend for X turns, and after the threshold, the xenonauts arrive to push back?),
Defend critical infrastructure like a highway or a military base (survive but minimize destruction),
Help an intelligence agency kill Cleaners (kill em all with friends),
Capture Cleaner supply convoy for resources (capture mission within X turns)