"The scourge comes about, not as an “attack” at all but the product of some cataclysmic event that ravaged these terraformed worlds"
There are Jardaan recordings aboard the Meridian command module, describing the Scourge as:
"The Opposition's weapon may cause widespread damage. All our weapons, our ships, will not be able to protect us… protect my goal."
"We need to disengage Meridian from command core, which will remain here to draw fire. Meridian contains all the work of the Jardaan. Nothing else matters. I will send it far. We can return one day. Continue the process of renewal. End of log."
"And so the Jardaan leave Meridian adrift and for the time being, abandoned their efforts in the Heleus cluster considering the event too catastrophic to ensure the Angaran’s survival."
It did not take much effort from a Pathfinder - someone who does not fully understand Jardaan tech - to operate/fix all the broken Remnant machines with relative ease up to 600 years after the Jardaan left (or vanished from) Heleus. It is not a difficult thing for the Jardaan to return and fix the cluster if they are still around. The far more "primitive" Initiative did it.
"The Jardaan have slowly all but forgotten themselves; now a cultish and dystopian society ensuring their reproduction and prosperity via conquest the Jardaan are no more...there is only the Kett."
As suggested by every Kett recording about this topic in the game: The Kett have no reproductive organs, their survival and/or evolution depend on the process of Exaltation, it's just how the Kett empire grows since the discovery of Exaltation.
There are many more Archons hitting different clusters in Andromeda, The Kett had already consumed hundreds if not thousands of species before Heleus.
Alec Ryder said at the start of the game - before the Initiative left the Milky Way, the golden worlds were perfectly fine. The game made it perfectly clear that they were deemed golden worlds because they were habitable planets and the Scourge happened during the trip. So...
1) Construction of the terraforming installations.
2) Creation of the Angara prototype race.
3) War with the "Opposition" - which could be another alien race known as the Jheln OR, more likely, the Jardaan and the Jheln are opposing factions in a civil war.
4) Scourge attack and the Jardaan people's mysterious disappearance.
5) Kett invasion.
... all happened in the span of 600 years while the Arks are crossing over.
Also to support this - something I already said: the Kett already finished off thousands of species prior to Heleus therefor the Jardaan really could not "slowly become Kett" over time and "forget who they were" in 600-700 years, that would be an absurd story if true.
"Elsewhere in Andromeda on the Jardaan home world of Sarhesen in the distant past the Scourge has spread all throughout the galaxy. Leaving the Jardaan in a state of a long suspended dark age."
"Maybe my theory is a little loose fitting but it makes sense to me. Why the Angara would be fitting to exalt but not many of the colony races."
The initial goal of this game's Archon in particular is to gain knowledge of Jardaan tech and dominate the Heleus cluster and beyond so they can slowly convert all the sentient species with genes that benefit the Kett's evolution, Milky Way races included. This is shown in the holo displays of the various captured Andromeda and Milky Way races on the Archon's ship, highlighting the species' genetic advantages - the Kett's interest is very clear cut as to why they do this. The idea of weaponizing the Meridian only came as a late game decision by an agitated Archon. In either case it's not about the survival of a dying empire due to the Scourge.
While I cannot answer the question, the theories so far can all be disproved by information available and mostly told through voice by SAM to the player in the game.
