Unit experience

[Tristuskhan]

Also I have merged fresh units to the depleted ones which understandably lowers the experience of the unit depending on the amount of new additions merged.

You can overcome this issue by merging the MOST experienced group in the depleted fresher recruits. Using veterans as the reserve to fill the gaps in your rookie's ranks makes the average quality of you lines rise, and is the best way to merge if you have a no-retraining homerule (as I do).