Full disclosure, I am playing this on the switch, not steam but I doubt that should have a difference since there is still one ??? unlock spot for the final ending. I have unlocked every other ending, good and bac and have followed the steps
:Midsummer
Go left
Rinné
She has her reasons
Thirteen years old
Setsuna?
The Ohara family's son
She's not related to the Oharas

..
However this option still eludes me. I even speed ran through the winter route again thinking I may have had to trigger a flag, to no avail

I swear I'll make Rinné happy