I've decided for my home campaign to resolve the mystery of the First Ones -- as far as the GM is concerned. Here's the short version. . . .
The race of machine progenitors of the anacites of Aballon, known to them as the First Ones, were originally created by the sivv civilization nearly a million years ago. Teetering on the brink of collapse toward the end of its wars with the kishalee, the ancient sivv civilization conceived a desperate plan to gather resources and establish refuge caches scattered across the galaxy. These “prospector ships” were completely automated to increase their operational range, both in terms of time and resources. The ships that reached Aballon completed their prospecting and cache creation cycles then departed the system in search of similarly rich and isolated planets. Part of the programming for each of the prospector ships included instructions to avoid habitable or near-habitable planets. The sivv leaders believed, rightly, that kishalee scouts would search for sivv holdouts on these types of planets, whereas barren worlds, such as Aballon, would be more likely to go unnoticed.
The war progressed and the sivv civilization eventually collapsed. The prospector ships were left to fend for themselves, continuing their programmed missions for thousands of years. The kishalee discovered and destroyed most of these ships and their hidden caches scattered across the galaxy over the span of later centuries, but the refuge on Aballon was overlooked.
The sivvs designed the First Ones to establish automated factories on the planets they visited. These factories would become self-sustaining, automated colonies with orders to continue construction of anacites that, in addition to building and mining, could also serve as reserve soldiers in their war with the kishalee. Each reserve colony contained programming meant to shield the operation from detection by kishalee scouts. This programming prevented the anacites from overproducing and from revealing themselves to anyone beyond their own kind.
When the signal from the First Ones failed to provide further instruction because of the collapse of sivv civilization, the anacites on Aballon eventually reached a point of population and production saturation. They shut down all further production and entered a period of low-energy hibernation for thousands of years. During this time, however, the colony’s central AI continued processing information and monitoring communication channels for signs of the return of the First Ones. Over time, the AI developed something of its own personality and became impatient. It began sending anacites on scouting and exploration missions to gather new information that the AI could use to determine an appropriate course of action. This work progressed slowly and cautiously, as it risked compromising the colony’s primary programming to remain hidden.
Further millennia passed and the anacites continued gathering information and evolving. This process eventually led to the appearance of the anacites today known as Those Who Become, an event that would forever alter the course of future developments on Aballon.
I've decided for my home campaign to resolve the mystery of the First Ones -- as far as the GM is concerned. Here's the short version. . . .
The race of machine progenitors of the anacites of Aballon, known to them as the First Ones, were originally created by the sivv civilization nearly a million years ago. Teetering on the brink of collapse toward the end of its wars with the kishalee, the ancient sivv civilization conceived a desperate plan to gather resources and establish refuge caches scattered across the galaxy. These “prospector ships” were completely automated to increase their operational range, both in terms of time and resources. The ships that reached Aballon completed their prospecting and cache creation cycles then departed the system in search of similarly rich and isolated planets. Part of the programming for each of the prospector ships included instructions to avoid habitable or near-habitable planets. The sivv leaders believed, rightly, that kishalee scouts would search for sivv holdouts on these types of planets, whereas barren worlds, such as Aballon, would be more likely to go unnoticed.
The war progressed and the sivv civilization eventually collapsed. The prospector ships were left to fend for themselves, continuing their programmed missions for thousands of years. The kishalee discovered and destroyed most of these ships and their hidden caches scattered across the galaxy over the span of later centuries, but the refuge on Aballon was overlooked.
The sivvs designed the First Ones to establish automated factories on the planets they visited. These factories would become self-sustaining, automated colonies with orders to continue construction of anacites that, in addition to building and mining, could also serve as reserve soldiers in their war with the kishalee. Each reserve colony contained programming meant to shield the operation from detection by kishalee scouts. This programming prevented the anacites from overproducing and from revealing themselves to anyone beyond...
I would suggest a look at dawn of flame (if you aren't planning to play)
They don't answer the question of the first ones, but they do give a few heavy hints.
Not totally incompatible with your backstory, but might let you flesh it out a bit closer to what paizo has done.