Building upon your sentiments, I believe verse 9:11 makes a significant point.
[9:11]
But if they repent, establish prayer, and give zakah, then they are your brothers in religion; and We detail the verses for a people who know.
Which is that the Belief in One God, Prayer and Zakah are enough to make a person a brother in Religion. So we can argue so all sectarian divides can be included as brothers in Faith.
The only thing that hinders me from fully accepting this, is that some argue that taking other sources besides the Quran can amount to shirk. In addition to this, verse 5:44 explicitly says that "whoever does not judge by what Allah has revealed - then it is those who are the disbelievers." Traditional muslim judge by other sources besides the Quran. I don't want to jump to conclusions but I am not sure this verse applies to them or not.
[5:44]
Indeed, We sent down the Torah, in which was guidance and light. The prophets who submitted [to Allah] judged by it for the Jews, as did the rabbis and scholars by that with which they were entrusted of the Scripture of Allah, and they were witnesses thereto. So do not fear the people but fear Me, and do not exchange My verses for a small price. And whoever does not judge by what Allah has revealed - then it is those who are the disbelievers.
So I have reached a dead end. One the one hand, those who belief in one God, pray and give zakat are considered brothers in Faith but on the other hand, taking as other sources besides the Quran is definitely a sin. But I am not sure if it amounts to disbelief/Shirk. Because if it does then how can those taking these sources be regarded as brothers in Faith.
Thoughts ?