Could relic xp bonus have 2 components or more? First thing that comes to mind is a flat multiplyer by faction standing. So if you have 2 relics, and the base is 10%, if you happen to have a faction standing of 100%, then you get the full 10% bonus. If you happen to be in bad standing with say 50%, then you get 5% xp bonus. If you are at 0% standing, then you get no support from your homelands, as you have been shunned.
My mind then wandered onto the idea of time based relic percentages like the above. It shouldn't be 'per capture' or 'per raid' it should be more global than that. Something that measures a 'unit' of relic capture per game hour and applies that as a negate for diminishing returns. With lack of battle for the relics, the holders slowly revert to a lower base level. (Removing it entirely promotes stagnation).
Faction A holds 4 relics = 4 units/hour
Over any X hour period if the average units is 7, then drop base XP bonus to 50%.
If any X+Y hour period is average units 7, then drop base XP bonus to 25%.
Repeat for cases 6,5,4,3,2 and code as needed
thinking back to the times when we used to go relic-happy , capture all the relics and reap the 50 % bonus was one thing; but with a % cut --- it does take less focus off of the relic insanity but at the same time ; if a group of faction members decided to cap them all they can still benifit well from the allocated % .
