Hi OtomeLover,
The ´secret ending´ is the extra ending on Adam's route, which you need to have selected a certain choice in the earlier dialogue with him in order to unlock. I assume since you have all the other CGs that you've already found that one ;)
You've definitely 100%ed the game. And again I'm so glad to hear how much you enjoyed it! Thank you for playing :)
Best,
Tibault