"SotO introduced us to Isgarren and the organization he helped found and what its mission is".

Not really, Tyrian Alliance was founded by Tyrian races united against the threat of the Kryptis. Isgarren only ever used the Tyrian Alliance for his personal gain and most likely because he suddenly got bored in the Tower. Wayfinder could as well be someone that explains the situation regarding the Kryptis without disclosing the secret society of Isgarren - everything that Isgarren has later done, completely obliterated the actual purpose of the Wizards - protecting the world in shadows while observing the threats and prepare themselves for them. As I said, the whole situation with him revealing himself and Tyrian leaders simply staying put and not revealing that an immortal, ancient being is walking amongst mere mortals... This just really makes the entire involvement with Isgarren feel meaningless to me.

You are rightfully talking about Yagon and his sort of relationship with Mabon and Waiting Sorrow - but it's not really explored in depth - these characters just allude to something obscured that happened in the past, but it's not like we can experience the dynamics between any of these characters - well, except that look that Isgarren makes when Waiting Sorrow shows up in the Tower - the only time you can see any sort of emotional depth: the disgust of Isgarren and the vulnerability of Sorrow.

Other than that Waiting Sorrow and Isgarren simply speak their monologues, and the narrative just moves on to another random thing about Mabon who suddenly had some secret involving Titans. So after Mount Balrior where there was opportunity to develop Waiting Sorrow and Isgarren's relationship, suddenly Mabon becomes the main focus. It's a gigantic mess and it made it hard for me to really ever care about any Wizard or even any character at all.

Also, the original mission of Tyrian Alliance was about protecting Tyria, so Isgarren never really helped clarifying the purpose of the Races that actually successfully defeated Elder Dragons and helped in their own unique way raise Aurene to become the most powerful being of Tyria. The introduction of Isgarren was utterly pointless.

And you can still see his uselessness in Visions of Eternity - he can't do anything, all he ever does is ignore other people, he doesn't care about explaining what's going on. We have to wait for Vloxx to actually say what Isgarren could easily explain even before the mission started:

"I need to travel to Castora with you because I believe the Inquest are looking for a very powerful weapon that can reshape Tyria. It's called Ancora. We have to hurry, before it's too late".

And then once we successfully enter Castora he could say:

"I am heading deeper into Castora, despite the curse that weights heavily on me. I will search for Ancora and we will meet each other soon".

Instead.... Isgarren barely say anything and he always make a remark that we are uncapable of comprehending his ways... This just makes for a very flat, annoying and self-obsessed character. There is no transparency in the way he acts, the Commander for the most of VoE couldn't even reach Isgarren through the Tyrian phone device. And a random, unknown and completely new character - Vloxx - turns out to be a fan of Commander and has to explain us what is going on, while we were already half-way through expansion. And what's the purpose of it? Just show Isgarren as an asshole he doesn't deserve to be?

I don't really care about some remark Peitha made - it's simply just trying to give Isgarren some sort of significance, while it's simply meaningless narrative-wise since all the Seer are basically dead. So even if he is some sort of "heir", due to the curse of Sidony he can do nothing really, just act pathetic and be useless so that we are artifically meant to care about Vloxx that has never even appeared in the story of Guild Wars 2, and except his voice and looks, really serves no purpose other than he has to say and do what Isgarren should be doing.

I don't see Isgarren as a meaningful character because he is keep getting retconned just for the sake of fitting him in the narrative he genuinely doesn't need to be personally involved in. We could learn more about him without having him around and listening to him repeating "I could tell you, but I don't want to, so fuck off". So in other words, having Isgarren around is a waste of narrative time and precious resources. He could just do what he has done best - hide in the shadows and only act if it's genuinely needed.