I think if your book club actually reaches the end goal and makes it to the “final” reward of 25 gems, then the key at the end of the points meter should unlock and give a bonus reward for completing the challenge. It’s hard enough to reach the end goal and my book club has only done it once, and part of the reason was to see if there’d be a surprise bonus at the end. But there wasn’t, and ever since then people haven’t tried to reach the end prize.
But I think if there was a random bonus reward that the key unlocked when the goal is reached it would be more motivating then just the 25 gems… especially because the only reason we were able to reach the goal was because so many people in our club had VIP status at the time, so the extra passes as a reward didn’t benefit those people.
So I think reaching the book club goal should make the key unlock for a bonus reward, but that reward should be something different each time to pique curiosity and motivate members to reach the goal.
But another thing is that if you do have VIP status, maybe gems should be the only rewards you get for the book club challenges anyway, because if you have unlimited passes as it is, then we don’t benefit from receiving passes as a reward.