Easiest it would be to make it score-related. Each deployment would have some score value, and if your last month or compounded, or some sort of hidden derived score (multipled by your numbers, for example) is high enough, you auto-win (with some random rolls involved). If no score value, autowin is disabled.
The same deploy-score mechanics can be used for surrender mechanics. As soon as average enemy Morale is say, 40 or less, at the start of their turn, they make a roll, only against double or 1.5 score. If they fail, you get a prompt with plea to surrender, ending mission with full capture if you accept, and continuing with some score hit if you don't.
If we add a time limit to board a landed UFO, we'll have a much smoother gameplay altogether.