Well unless this is linked to the individual experience of each citizen soldier: advances in fighting capability/ranking up would then coupled to an individual gathering capability decrease. Newly trained uniots would have full capability (for a CS) decreasing with each fight. In the end you woudl have an army of fighter pros that are not good for any gathering anymore while the newbies can very well help in the economy but are less powerful in battle. Sound interesting to me.

If it was for SP only, I would recommend this as an optional setting to chose from. For MP games I am not so sure.

I geniunely recommend you to learn how to play. Top players in the game don't use any automated training system. If you think that recommendations of doing your own mod is "humilliating," then will be faster for you learn the game mechanics than make a mod. GG.

Another idea somewhat related to this. Farm upgrades, instead of giving you +x% faster gather rate, allows you to put one more farmer per Field. E.g. Fields start with a maximum of 2 Farmers, and each Farm upgrade increases the maximum by 1.