I hope you continue to test ideas. As you mention, you need to hit the right model for freemium to have a chance of working out.
The experiment of credits seems potentially weak. As a user it feels like a pain.
“Now I have to worry about top ups and credits? I just want to read some stuff man!”
What about a simple lock/unlock? First item free only! What about ads/no ads (although I dislike that model)?