I think that each credo progression should be changed to points instead of quests.
For example, instead of 10 quests you would have to gain 10 points.
You could gain points by completing quests, first quest each day would give you 5 points, second 3, third 2, and the rest just 1.
This way you could still grind all credos really fast if you want to. But you'd have chances to get credos even if you only played few hours a day, albeit slower.
There are many ways to implement the same idea, like some boost that charges over time (like hunger?) or such.
Core idea being:
The gain from time used in game shouldn't be linear.
Because if its too easy, then people who play a lot will get bored fast as they've completed everything. But if everything is made time consuming to get, then casual players will never get anything nice.