Currently, melee units are weak compared to ranged units. The pathfinder is still problematic and ranged units have advantages with that. Melee units do not have the distance and pathfinder in your favor, so it seems fair that they have less xp requirements for promotion. This should improve the melee units without having to change other attributes.
This idea was put by @feldfeld (I don't remember which diff)- Ranged unit requires +25% experience to promotion.
It seems better to do this through tech.
Affect "melee"ranged" seems more appropriate than "infantry melee", "cavalry melee", "dog".ranged", Look also D2879"cavalry ranged".
D1977 is very important and need be committed.