Did Nyrissa accept Briar when you offered? If she rejected, then you didn't romance her. And to unlock the true secret ending, I heard you have to finish all relevant curse research project via kingdom system.
I'n neutral about ending choices.
Because I romanced Tristian, Nyrissa rejected briar. This meant at the end of chapter 6, I had two choice - either offer her to join me or let her go. Letting her go ends the game in chap 6, which I found a lot more satisfying - now my kingdom is safe, and Nyrissa gets what she deserves (Lantern king is such an ass). If I ask her to join me, then chap 7 starts. Nyrissa fought alongside me against Lantern king at the end of chapter 7 without bugging out. I also asked about Nyrissa's fate to Lateran king after the fight and it didn't bug out.
At the end of chap 7, I was offered three choices - I tell both to leave me and my kingdom alone from now, or made immortal by either the Lateran king or Nyrissa. None of them felt right because I didn't want to be immortal and owe neither Nyrissa or Lateran king. But to just walk away means, people in Golarion suffered due to my arrogance (same result as letting Nyrissa go at the end of chap 6, except so many people died because of my choice + the wild hunt frustration), which kinda gives the gist "know your place as a mortal or suffer the consequences". I kinda liked that, you don't just punch on with Demi-gods and have easy time getting away with it.
I'm with you for hoping Owlcats will fix the end game. They really need to balance out the castle, like half the number of wild hunt waves or something. It's tedious and rage inducing for no good particular reason. Also the bugs as well - can't imagine your frustration to go through all that at the end
Are you going to attempt to unlock the secret ending again, Maylander? I decided not to bother, seems too much of effort, going to stick with finishing the game in chap 6 in my third playthrough.