Guardians were taken out by the AI race. Not thargoids, of which they seemed to be less at risk of being destroyed by than we currently are. If they were running from someone or putting out a backup plan for survival, it would be against the AI. Which sounds a bit futile, plus it happened very quickly with little time to respond. The AI race wouldn't have allowed any living guardians to survive. Granted, the mars artifact could have been planted long before their end and even before the ai race was created.
Perhaps raxxla is an AI race station or jump gate. Their technology would have advanced well beyond what it was millions of years ago when they took out the guardians and since their primary goal was wiping out the thargoid threat, it would appear that they are no longer in the galaxy or have other priorities now. Survival of the fittest and all and see who can figure out the way to reach their current location and join them on their journey to infinite.
I think early human explorers got hints from things they found about raxxla. It also stands to reason that these hints didn't provide a location or that location would be in the hands of super powers and/or mega corps. It would really make more sense if nobody has ever found raxxla. We have no evidence of advanced technology suddenly being made available.
The biggest plot hole we have in the elite universe is the permit system. How are these permits enforced? Who controls them? How have they not been fragmented by opposing super powers? Who has declared that entire sectors be off limits? Why? They really dont make much sense at all lore-wise.
But if they did have a lore reason, that organization would seem to wield incredible power. Controlling which systems 99.9999% of the human race can access. Really i dont know how such an organization can exist within the elite universe. The only thing that would make sense is that they are leveraging something over any power or company that would wish to take that control away from them. And what could that be other than something as awesome as raxxla?
The traditionalist Guardians were (according to Ram Tah, but we were warned not to believe everything we are told by characters, we have no way of independently verifying his decodes) wiped out by their AI Constructs who persuaded their war machines to rebel.
Before that:
1) the technology-oriented Guardians were exiled so may be somewhere in the galaxy (or whatever they evolved into) so they might have dropped the Mars Artefact
2) The war machines were pitted against the Thargoids, but the war was fought to a standstill. Don’t think the AI per se were antagonistic to the Thargoids.
You said “I think early human explorers got hints from things they found about raxxla. It also stands to reason that these hints didn't provide a location or that location would be in the hands of super powers and/or mega corps.” This is supposition & there is no evidence for it. We have very little evidence so I’m not against supposition (hypothesising) per se, we all do it, though hypotheses should be tested against in-game reality. However if I understand the timeline correctly the evidence we do have suggests the Raxxla rumour started around the time the Mars Artefact was found (& Achenar was settled). The Feds have never revealed what the Artefact was or anything about it, so it may be that Holdstock was correct and Raxxla is being exploited by unscrupulous men.
Permits are enforced by code within our fsd drives (made by Sirius Corp). They are set by the Pilots’ Federation, who clearly don’t want us to go into certain areas of the galaxy (yet). They also control Universal Cartographic so have oversight of anything we find and wherever we explore. Clearly this is an exceedingly powerful organisation. I think it is telling that they make use of a triangular motif which is very similar to that seen at Guardian sites & on Guardian technology...