Yeah, it's a very well done and very fascinating relationship and also very sad. Nie Mingjue has a decisive role in winning the war but you see what it does to him, and you also see what it does to Nie Huaisang to choose not to fight the way his brother does.

I'm really not surprised that The Untamed is such a huge fandom, it has a lot of very interesting characters and relationships, and most of them can be interpreted in different ways. So it's possible to find more or less whatever character and relationship tropes interest you. A lot of the characters have complex story arcs and you can see how very many of them could have the story told from their perspective and it would be a very different (but still very good) story.

But yeah, there are four main sects (not counting necromancy and wandering cultivators) and there are very interesting characters in all of them. If you want to get a sense of how big the world is, you can poke around the wiki:
https://modao-zushi.fandom.com/wiki/Grandmaster_of_Demonic_Cultivation_Wiki

The relationship between flute-playing necromancer and his stoic monk boyfriend is nicely done, but there's so much else going on that interests me more so I haven't really focused on it.

ETA: Here's another Nie brothers vid that I like a lot
https://www.youtube.com/watch?v=hZQBkmnH_Oc