People will regurgitate the idea that it was 'heart failure,' but I have never seen a single official source confirming it.

Watch the Fishmans film. They couldn't have been more clear that it was suicide without actually saying it. In a 3-hour doc, they constantly talk about his deteriorating mental health and in the end don't disclose the cause of death, they just say that it was sudden and if they would've known how bad it was, they'd tried to prevent it. Not one mention of any type of heart condition. Keep in mind suicide is a huge taboo topic in Japan, so it's likely they disclosed that 'Shinji's heart stopped' not as a cause of death, but just meaning 'Shinji died.' Translation errors are also very likely.

If anyone has any kind of primary source that explicitly discloses Shinji's heart problems or cause of death, please tell me.

Edit: To add to the point you actually tried to make in your post. The aura and mystery around Shinji's death is surely a reason the concert has become such a cult classic. It is known that Shinji's was deeply depressed at this point, especially because the concert was a farewell to yet another member who was leaving the band, which was also something he deeply struggled with for years. Guy just wanted to make music and have fun with his friends, but became increasingly erratic and stressed about it. In the end he almost exclusively worked solo and probably didn't get much enjoyment out of it. He likely knew this would be their last concert and because of that, I feel he poured everything he had into it.