So just to clarify: I come into a settlement once and recover two relics, I get the 50 credits. I come back later with two more and get both the tier three and four rewards for a total of 200 credits and 3 fame?
Well you seem to have made the flow of relics limited enough not to make it too extreme, and thankfully it seems to line up nicely with my play-through that finishing the Relic Guild's quest line will get me about or to ten fame and end the game, so it doesn't seem too game-breaky!