Hey! If that's okay to ask, how did you get the true ending even? I used all 4 remembrances on the table right after I defeated the Idol, but nothing changed and I just got the common ending. Have you maybe went to the Void (Judging from the game files there are void enemies so maybe there's a void location) or to the basement of the chapel in Auradon?
I hope you found it cause I'm a bit late with the answer and feel free to tell me if I'm wrong because I dont really remember but I'm pretty sure it had to do with using the 4 remembrances on the table and then ascending your soul twice, gaining the item "Soul of Elsbeth Krähenlied" or something
Pretty sure you get a non missable soul item that's equippable in the early game, before some boss. Then late game after getting the first 4 remembrances you can find an altar that lets you ascend your soul. you need to find two of these and ascend twice.
Think thats it. Ending isnt much just a new picture and a line of text.
Good luck!