I thought the same thing at the same point, and I wondered why the idle activities were becoming more and more useless. Then I discovered that if I actually went back and finished off the first couple of activities, instead of maintaining proportional spending, then more activities became available, the manual activity shifted from "walk" and became stronger, and the pace of gameplay became reasonable again.
So I don't really think the balance itself needs all that much changing; rather, there needs to be better signposting that completing all 100 levels of the early activities is a thing that should be done when possible.