Well, the way I've been handling it (lately) is by including Grandfather and his descendants... who were considerably more numerous than in the "official" line. At least two additional generations, meaning there were at least 160,000 "Ancients"... who are actually one of the "interim" civilizations. (There have been at least four large-scale galactic civilizations occupying the volume the Third Imperium and surrounding polities currently hold, of which Grandfather and his followers - and their uplifted servitor races and synthetics - are merely one. All of these are far enough separated in time that there is essentially no meaningful communication, even one-way communication, between them, by which I mean that, by the time a civilization or group of civilizations begins analyzing the remains of a Precursor Civilization to the point where more than a cursory history is laid out or a very poorly understood prototype copy of "super-tech" is being put into production, the newcomers are already starting to verge on the same order of development as those they study.)
The true Ancients in this portion of the Milky Way actually date back considerably farther than any of the theorized Precursor Civilizations - Grandfather and his relatives reigned, IMTU as in the OTU, on the close order of three hundred thousand years ago. There have been at least two, possibly three intervening Precursor Civilization, and there is probably at least one which preceded Grandfather by about another two hundred thousand years. Prior to that, however, it was a long step back to the Ultimate Precursors... something on the order of fifty million years. And the Ultimates were not alone... during their explorations of the galaxy, they discovered a contemporary race, one which may have been of equal power, or may have been even more powerful. These nemeses were located somewhat Spinward of the current campaign area and considerably Coreward, and were distant enough that none of the 3I's contemporary empires have reached their territory - both volumes were demarcated by heightened concentrations of lanthanum, allowing Jump to be locally developed, and much of the space between was nearly devoid of usable amounts of the stuff. In any case, the Ultimate Precursors and their contemporary rivals went to war against each other, a war of intended total genocide.
The Ultimates lost.
For the next several million years (how many million is uncertain), the Rivals swept through their enemies' former territory in war fleets the size of which has never been seen since, but on irregular patterns, since the volumes were orbiting the rest of the galaxy at different rates. They searched out their enemies, destroying them as remnants were found. They destroyed any new civilizations which began to emerge, as soon as those civilizations became advanced enough to make themselves detectable.
And then the raids stopped.
Where did these Great Enemies go - if, indeed, they went anywhere? Do they still survive, somewhere, or did they leave heirs? Do any artifacts - from either side - still exist? And what might be possible?
If the characters decide they're interested in pursuing "Ancient" lore, they may discover part of this backstory... and they may - just may - encounter some form of remnant element of one of the Precursors. Yes, there is at least one survivor from Grandfather's empire, possibly more than one, sequestered and watching... because he is at least somewhat aware of the galactic history which includes the Ultimates and their enemies, and he knows that nothing from this region of the galaxy ever defeated those enemies. And that he knows of no reason that the raids might not resume if civilization is once more founded in this region...