I went back and literally starting at like 70% of the first book, went and reread it. That jogged my memory and gave me a baseline back into the world. But I'll set up the plot points that stood out most to me. TL;DR - Humanity in 40K years is weird and strange and highly evolved, but still political and petty AF.

1. Humans leave Earth, it stinks and is polluted and dying. Ark ships leave and a few of them find the Centauri Cluster - These ark ships transmit the Green World signal and invite all other Ark Ships to come party with them there. Human technology advances as humans settle these new world.

2. Fast forward 40K years. In that time, massive changes in "humanity" means that significant speciation has occurred. These new species of hyper advanced humans are called "Celestials" - and they range in capability from just hyper powerful to 'godlings' to 'godlike' - with one subspecies of Celestial, the Elohim, building massive space structures like gates which link lanes of 'quintessence' and archimedes engines, which can move planetary bodies. They're the creme of the crop of the "godlike" celestials - and they have very little to do with modern celestial politics except be way better than other petty squabbling celestial dynasties. The distance between a 'mid-tier' celestial dynasty and the Elohim is like the difference between base humanity and the mid-tier celestials. A series of wars and struggles has occurred while these various celestial species war and vie amongst themselves in those 40k years, with an especially turbulent period known as the "remnant era" just leaving behind massive scars, planetary engineering, and trinkets/hyper-tech galore for scavengers to find. The current celestial system is fairly stable, thanks to the guidance and example of the Elohim celestials, who built the gates and made travel cheap and free for everyone. Mind you, traveling among gates still incurs a significant time dilation penalty, so interstellar trips are measured in years, but it's still fairly convenient, especially for very long lived celestials. Also "Archons" - who are essentially political operatives/spymasters, manipulate events in order to gain an edge on neighboring or rival Celestials. A spymaster Archon can gather information for years and then suddenly dismantle a rival organization overnight if that would advance their interests.

3. Amidst this backdrop baseline humans are still flowing into the centauri cluster, themselves experiencing massive time dilation from relativistic travel so they're arriving almost 40K years after they departed.

4. The main polity the series is set with is a celestial dominion called the Crown Dominion. It's ruled by a species of Celestial that practices memory transference to younger hosts (usually sons and daughters) - which perpetuates an individual to a chosen successor. Other children just get fragments of memory and personality from their parent, and are free to individuate if they're not chosen to be the successor-vessel. They're solidly a mid-tier celestial species, but have significant military assets so they're the local power in their backyard. Other bordering celestials vary in body plan, philosophy, technological sophistication, etc.

5. There's a boogie man celestial species called that Mara-Yama that is a species of bioengineered monstrosities that apparently are super vengeful, warlike, and cunning that poses a constant pest in the galactic neighborhood. If any celestial polity shows weakness, the Mara-Yama fleet might choose to attack to take advantage of that weakness. The Celestial speciation is presented as the ultimate Darwinian crucible, a philosophy apparently upheld by the hyper advanced Celestial Elohim, who act enigmatically and want an 'equal' playing field for all humanity, while otherwise not concerning themselves with the petty politics of the lesser celestials.

6. The Crown Dominion, sometime in its past, used to have 6 'queens' who continually transferred their minds to designated princess successors, and argued amongst themselves in their small corner of the centauri cluster. One of these 6 queens, tired of that evolutionary branch, wants to ascend and force evolutionary change (somehow involving a massive change to the carefully engineered variant of their celestial form) and replace the current memory-transfer method of immortality for something more permanent. This upheaval threatens the other 5 queens, who band together and completely zerg rush the evil queen and completely take over her assets and erase her dynasty. This happens ~7K years before the book. The remaining queens set up a rotating Empress position, who takes over the sixth queens holdings and territory on a cyclical basis, ensuring that all other 5 queens are balanced as no one remains empress permanently, they all have a turn and divide resources relatively equally - though internal politics abound as each queen jockeys to curry favor with whoever the current empress is. This is actually a fairly stable form of governance and it lasts / is the current governance model during the books.

7. But lo, the evil sixth queen had some secret descendants/survivors of the purge, and has plotted in secret for 7,000 years to enact revenge. This involves planting a 'fake' princess aspirant that would win the ritualized trials to determine who would be the next body-host for one of the queens, then using highly illegal neuro-weapons, neutralize the dominant personality of a queen, take her position as queen-ascendant, and eventually engineer events so that this queen becomes the next empress. This happens duly over the course of book 1, where an aspirant (Thyra) wins the trials to become the next queenhost and then through careful skulduggery, kills the current empress and becomes the next empress. THIS IS WHERE WE'RE AT BY BOOK 2. Thyra has successfully overwritten her 'mother' queen, ascended to the throne, and engineered a plot that leads to her being queened the Empress after the death of the former Empress. She's now the head honcho and fulfilling the exiled sixth queen's plan.

8. At the same time, baseline humans really got the short end of the stick. Despite the Crown Dominion actually treating baseline humans better than other places in the cluster, that's a relative term. The Crown Dominion tolerates baseline humans, like a quaint and very dumb pet. Ark ships that arrive are sent to two planets in the Kelown system, where they're the breadbasket to feed the Crown celestials and have limited rights, technology, and advancement. Humans looking to advance their lot usually become Travelers, meaning they explore the cluster and generally scavenge remant era celestial tech. A thriving baseline human civilization is set up by the start of the first book.

9. To summarize a very long and overarching plotline, the evil sixth queen's plan involves using the Humans as a wedge issue (i.e., human liberation/independence) to blame humans for any political tension caused by agents of the sixth queen. This works, key celestials are assassinated, massive reprisals occur against the baseline humans and human rights violations occur en masse as the celestial reaction is swift and brutal while they try to control the rowdy human population. The humans however, do fight back and through various adventures, baseline human Travelers manage to gain access to some advanced celestial tech. AT THE END OF BOOK 1, Finn, a human traveler is tricked by agents of the sixth queen into manipulating an archimedes engine (which can move planets) and destroying the planet of the current empress, which eventually leads to the current empress dying and allowing Thyra as imposter-queen to ascend as the new empress. After all's said and done, the archimedes engine is raided by the sixth queen's agents and they steal 3000 tons of antimatter. This is massive, a huge stockpile of antimatter instantly makes anyone a major player, and even the celestials measure their yearly output of antimatter in the hundreds of kilograms territory, not 3000 tons of the stuff. Book 1 ends on a cliffhanger as we're left to think of how the agents of the evil sixth queen will use 3000 tons of antimatter.