I don't believe there is, or can be, such a thing as a stable multi stellar society. Barring FTL technology, it's impossible to correlate activities or maintain coherent relationships when every interaction takes years, decades or centuries.

What you have are individual civilizations which may or may not, as you mention, decide to invest the time and resources into interstellar travel. But that investment isn't guaranteed. We've had the technology to send out interstellar probes for decades, yet we haven't committed to sending them out by the hundreds or thousands. We could have colonized Mars or the Moon by now, built Orion starships, but we haven't. Technology doesn't govern the advance of space travel, politics does.

>How does that uniformity of choice get enforced across time and space?

I think it's just a matter of probability. I don't believe technological development, much less to the degree of having a space program, are an inevitable result of intelligence or the presence of civilization. It's easier not to have a space program. It's easier, if you have a space program, to only explore your own solar system. Any number of issues, such as not being near enough to a colonizable star, not having sufficient energy or resources, natural disaster, giant ants, robot uprising, superflu, etc. could keep a civilization from starting, much less maintaining, a project of the necessary magnitude for the necessary amount of time.

And then you have to survive the trip, and actually succeed at colonizing another planet.

I think it's entirely within the bounds of reason for every civilization which has attempted interstellar travel to have failed up to this point.