basically required reading so that we can have a community with a fixed starting point
What exactly do you want these "scriptures" for? The culture is not the "doctrine". Let's not conflate the two.
If you want to join the culture, the only way is to interact with it enough to let it rub off on you. This takes time. HPMOR is a great introduction to it, but the other sources you mentioned also work and they're not the only ones. It doesn't particularly matter where you start, as long as it holds your interest long enough for that osmosis to happen.
The "doctrine" is perhaps easier to distill, but the distilled version is not always easier to digest. Inferential gaps are a real problem. The Twelve Virtues of Rationality is such an attempt, but it's a terrible starting point IMHO. They don't make sense unless you already understand the principles behind them. They're better at helping you remember things once you already start to "get it".
I put "doctrine" in scare quotes because even calling it that puts it dangerously close to certain failure modes we warn against: "If you speak overmuch of the Way you will not attain it." I feel like saying "our doctrine is that we have no doctrine", but that isn't quite right either. We have two core principles, plus some key insights that most of humanity currently lacks. The rest follows from that.
The first principle is "self-honesty" or "Epistemic Rationality". Study not the capital-T "Truth" which the deluded and the deceivers will say they already have, but the methods that lead one to it. I have found no better illustration of the correct attitude of self-honesty than the Litany of Tarski, which is a template you can apply to any claim, especially claims one might be inclined to take personally (e.g. "God exists"):
If the box contains a diamond,
I desire to believe that the box contains a diamond;
If the box does not contain a diamond,
I desire to believe that the box does not contain a diamond;
Let me not become attached to beliefs I may not want.
The second principle is "rationalists should win" or "Instrumental Rationality". What wins is how we define "rationality", regardless of your preconceived notions. "Rational" techniques that don't win are suspect.
Many subcultures claim to have insights that most of humanity lacks (and some even do). Which of ours are the most important is less clear-cut than the two principles.
I feel that the links in Raising the Sanity Waterline, and also The Fable of the Dragon-Tyrant, and Beyond the Reach of God are especially important and could serve as your "rationalist scriptures", but that's my opinion.