May I suggest lowering the multiplier percentage? With 28 days and three multipliers of 5%, the total maximum for the multiplier is 5.2... I personally find it too much for such easy to complete things which are even unrelated to the main subject of the challenge - the aspirations. How about 1%, 1% and 4% (one percent for each thousand)? This would lower the maximum multiplier to 2.68.
Or maybe we could use the "one percent for each thousand" principle and give 1% for each full thousand earned/donated? I think that the idea of the multiplier in challenges should be finding the perfect balance between the main goals and the secondary ones. This way, a player has to decide which is better for their, let's say, goofball sim - to keep writing and earning more royalties, or to spend this time on completing aspirations.