No, it’s definitely achievable in the version of the game you’re playing
All you need to do is:
1. Achieve max affection for all the heroines (I think it was 13 for Mom, and 11 for everybody else)
2. Sleep with all the main heroines enough times to make them obsessed/addicted to you (this includes Isabelle)
This isn’t a proven fact, but my estimate is that you need to have sex with each of the individual ladies at least 10 times to get the “true best” ending (and, by “sex”, I mean vaginal penetration)