What are the beliefs of Islam that are shared across all (or 99.9999%) the varying sects?
What ever are not disputable among the sects is shared across all of the sects.
Is there anything outside the 5 pillars (or are there things within it that are disputed)?
The Shia and Sunni both agree on the essential details for the performance and practice of these (5 pillars) acts. (Source 5 pillars)
The core beliefs of Islam,
1 There is only one God whom we cannot see now and is above the heavens ruling the kingdom which he created.
2 There is no other God besides him which means he's got no family but only friends and enemies or you can say his creations.
3 There is no image of God. If there were to be one, it clearly contradicts the first rule. Which also clearly means and is interpreted in Islam that there are no humanly avatars of God.
4 Remember the Friday prayers without fail.
5 Honor your parents and don't disobey them
6 Believe in the flashback.
7 Obey God and Obey the messenger or in other words Obey God and follow the messenger or in other words trust God completely and trust the messenger completely.
8 Prophet Muhammad (pbuh) is the last messenger of God.
9 Believe in the day of resurrection, the knowledge which only and only God has and the decision will solely and solely be of God on that day.
Last, but not the least,
10 Remember God with full devotion in the way which the prophet taught us or in other words, establish prayers five times a day else, God will be make devil as our friend and you know what happens next.
The rest, Ah! it was getting too big, so.... and somethings which I left life this are misinterpreted. You know it.
Islam has a holy text and varying interpretations
Finally, the authenticity of Hadith books is disputed, but never ever and again, never ever about the authenticity of the Book of God.
Hence to conclude, the things which are perfectly and literally clear in Qur'an are not at all disputed. But the varying interpretations of Qur'an arise due to the indifference caused by the belief in Hadith and their reliability.
May the creator guide us all.