OK. I know this might not be a supported idea, especially since it does involve more work on the development end, but perhaps a between-mission training mechanic would be sensible. Sure, sometimes people get better just through experience, but often they need to both train, and apply themselves in the field to improve significantly.
Potentially gaining strength by two points per a mission is probably too fast. On the other hand, a 30 kg hard cap is somewhat unrealistic for soldiers. (reasoning: a number of my (quite nerdy) friends, who don't work out or exercise other than our regular walks about town, can lift and carry more than 30kg in the form of objects carried. )