How about simply restricting prestige increases to real, pvp combat? After all, being an infiltrator or courtier isn't all that prestigious.
It would cause problems for Priests, though (which can be highly prestigious in my optic), as well as for prestige already gained by various characters.
I don't think prestige should drop at any rate, though. You'd go to war once, or twice, and then see your stats drop again? That would work as a detriment once more. Let's keep it positive
Or we could put in a new stat. Something on the side. Or perhaps more active advertisement of those realms that are actively fighting, so they see themselves rewarded with more nobles? That kind of brings us back to realm achievements, though.
Lastly, I very much agree we don't need any more individual achievements. Fame hasn't been mentioned yet, even (I consider it to be at the player's level - cfr. it's severely limited influence in-game [at least for me?])