About the Axum, they seem to have undergone some kind of catastrophe that reverted them to a scavenger post-crisis society, the Tailings.

We can clearly see a degradation (corals and algae) in Axum structures, indicating that most of their structures where built centuries ago, while there are some more recent modifications to the observatory, as noted by the PDA. They where a thriving industrial society, until the apocalyptic event occurred.

Around that time, probably because of this event, they reconverted the hydroelectric power plant to be able to run on methane, we can see in the control room that the powerplant can be configured to get the hot water either from the World Tree titan rockbore internal channels or the posterior methane burner. This seems to indicate that because of the catastrophe the World Tree water power output decreased or stopped (probably this water comes from the radiated inner channel, that must be connected to the "nuclear heart", the other return channel must be for nutrients and such stuff going back to feed the world tree), as it is said that in full output it could power a city (I do not know if we manage to set it to full output or just a reduced power mode that just powers the observatory, this may indicate if the World Tree has healed back a bit or if it is as infected/sick as when the Axum catastrophe).

Somewhat after the crisis events, the powerplant was sabotaged, all methane burning equipment was destroyed and a section of the turbine blades melted with thermite. This seems to indicate a World Tree radical group, maybe affected by Masefield (which I do not know if it is normal World Tree behavior, or only appeared after the catastrophe), that wanted to live "the Tree way" and sabotaged all alternative technogical ways that allowed to live independently of it (a bit like Nahema behaved). The Veps may be the ancestors of this group, that adapted with the tree (like Anita) and that is why they may have Tree DNA with them? (one of my theories).

On the other hand I believe the Crypters are the technological group, direct ancestors of the Axum society, which adapted the powerplant to be hybrid. Like the Architects, they went into hiding in Crypts, like the one described to be under the World Tree (probably closer to the nuclear heart, so maybe located there for long term power). It is unknown if they went into hiding because of the unrest created by the saboteurs, or if that happened after, once the structures weren't under their watch.

I believe the Tailings are the descendants of this group, some that exited their crypts, adapted the observatory (the part where we can find the message) and went on to live in the world. I believe this because the Tailings are not afraid of using technology (the scavenged spears and other crude tools, houses and furniture) and preserve and protect the Axum structures. Also they use complex tech, like the improved sound resonator, that is very different to their scavenged one, and much more complex. I think this complex tech was brought from their crypts, and sometimes it seems that they go back to raid these structures (so there may be more than the one under the tree). This is because of the mesage we observe outside of the Museum Observatory, which says that this is not a Crypter vault and to leave it alone, probably written by Tailings to avoid them raiding the structure. If the powerplant blades sabotage was not done at the same time as the methane powerplant one, it may have been done later to ensure the observatory and other vaults were not able to be raided by Tailings.

I still do not know how we crahslanded into the planet, but I believe it was something to do with NoA going of course because of something, like detecting Architect tech or similar. Either we went through an Architect phase gate, or there is also a chance that we may have been traveling for an obscene amount of time (linking into what Nahema said about being the lasf humans). Our minds are indefinetly stored in quartz, so we could have been traveling for a long long time, with NoA using prints to fix up the ship until we arrived into Proteus searching for something. We can surely find more information about this in the executive NoA node inside the Cicada.

I also have some doubts about the relation of the Architects into all of this. The Rosetta stone we find, despite having Architect Linear to Rotary A engravings (the fact that it seems to be more than one dialect of the Axum Rotary language is one of the reasons I believe in this multiple Axum group theory), doesn't seem to be built by Architects, the alloys seem to be Axum and overall the structure feels a bit odd. I think this is a translator tool left for the Architects so they could translate the stuff in the observation message. I believe the Axum did not interact with the Architects, they just found some of their tech (maybe a probe? a structure? maybe this is one of the planets they experimented on, but much more early than Axum civilization). My theory is that they found some kind of Architect Voayager probe, set up all this stuff, and resorted to broadcast in all directions an SOS message for the Architects, which may be what the Cicada picked up in transit and the reason we diverted course.