I will say that telling/not telling Umeko and trusting/not trusting Takako are two of the "bigger" decisions in the game. However, it is possible to get Ending 8 while not trusting Takako. Glad you're enjoying finding the different endings! And seems like you might've been close to Ending 18, which is one of my favorites.
We do have a guide if you decide to end up needing help and want to consult it (over on STEAM).