You’ve got the important foundational elements. The other big event is the Horus Heresy, where about half of the Emperor’s sons fell to Chaos and started a huge civil war that left the Emperor in his current half-dead state, but the imperial bureaucracy keeps on chugging.
The other thing to keep in mind is that the world of Warhammer 40k has been evolving over decades of novels, tabletop games, and video games made by multiple authors. As a result, the canon is vast and occasionally self-contradictory.
If you want to know more, I highly recommend the Ciaphas Cain books (novels with a comedy streak) and a silly fan series called If the Emperor Had a Text-to-Speech device.
What? This is a press release from the California Attorney General literally taking about what they are doing in court right now.
How do you go from that to “thEy’Re DoiNg NothIng”?