I actually like a "skill shareware" (my made up name) model in puzzle games. When you have lots of levels, you split them into groups. You can only advance to the next one if you solve all previous ones perfectly, (3 stars / 100% / some other metric) or pay for full unlock.

I've never been on the other side, so I don't know how well that converts. But as a customer, I love the idea. "Euclidea" does this for example: https://play.google.com/store/apps/details?id=com.hil_hk.euc... They also have IAP for dark mode.