This is interesting - how do you know that they would for a fact develop at a cultural crawl? Have we seen an STL galaxy-spanning species develop? Not saying you're wrong, it could certainly be, but I'd like to hear why you say it's a near certainty that they'd develop slowly rather than at the same rate or even a faster rate than human civilizations as we know them - why "must" that be true?
Our own experience with culture is one of rapid shifts. A hundred years ago, our lived experience was fundamentally different from our lives now; we assume that a hundred years from now, our experiences will be different still.
For us, an interstellar, STL culture is unimaginable. Our experience shows that we would lose much of whatever commonality we started out with in just a few decades of isolation.
But if we assume that the Ancients could built an interstellar society that way, then that must mean that their culture is much more static than ours is. That they can remain recognizably "Ancient" even if they've been out of touch with their homeworld for thousands of years.
To me, that suggests that they are slow to develop new ideas, and somewhat reluctant to adopt them when they do. They probably took a long time to switch from newtonian physics to Einsteinian, and probably even longer to adopt the utter mind****ery that subspace would have to be for them -- but when they did, they could disseminate that tech quickly because they knew that all the colonies out there were still thinking roughly the same way and still speaking the same language, something humans could not.
This sounds very much like the depiction of 'The Race' in Harry Turtledove's 'World War series'. They have a 3.5 system STL empire (the .5 being ours, since they were only able to successfully take over Earths' southern hemisphere) Their society was hyper-conservative. They fear change. Indeed 'that would be a change'
is possibly the worst sentence a member of the race can utter. They revere the spirits of past emperors and their imperial lineage goes back 100,000 years or more, so their society is very stable, but also very slow to adapt. A comical example is when they invented TV. They first allowed only a small isolated population to experience it for about three generations
. Only after that experiment showed that TV was not disastrous to society did they allow it to the masses. (Considering how bad Reality TV is nowadays, they may have had the right idea
Their hyper-conservatism is linked to their development and geography. Their home world has no natural barriers to movement - no oceans, no mountain ranges etc. As such their tribal ancestors were pretty homogeneous, with little in the way of tribal conflict. So their society didn't fracture as much as ours did, and the lack of conflict meant they didn't need to advance rapidly. so hey didn't.
The other two sentient species they have encountered and taken over were very similar, so they assumed all life was like that. So, when they sent a probe to earth during the medieval period and saw knights on horseback being the most advanced weapons they naturally assumed that would be the same when their invasion arrived in 1940. We came as one hell of a shock when they saw we now had tanks and spitfires ;-)