At this point, I'm convinced there is no true end. I want to believe I'm wrong, but even with the walkthrough, getting the 'true end' is way too difficult. It's a shame, because I love this game, but it's been 4 days and still no one knows how to get a better ending than that dickbrain Calis possessing you. Yaaaaaaaaay.
To be precise, for me, Vera and August are both at SSS, Dulcis and July SS, and Sekani and Lusida at S, and June at B, where she maxes at A. All of this somehow isn't enough to get the true end. I'll mess around with the other two paths for a bit, but I'm thoroughly convinced that based on the posts in this thread, I'm probably not going to get anything, sadly.
EDIT: Welp, I did get two extra small endings by going a different path at the start. Not the true ending, sadly, but it's something.