Question: Is it conceivable (with the aggravating factors proposed
below) to see this tendency become global in the future, and the
population ending up decreasing up to extinction level?
No. Not because of the factors you mention, anyway, specifically:
- Humans go live on a range of planet around close stars, thus the population becomes more scattered.
I don't see how this would have any meaningful effect at all on human population on Earth, except for the reduction on Earth of people who leave. So unless Earth becomes a horrible place to live, so that most people want to leave Earth, I don't see this being able to endanger the survival of Earth's human population. That's quite possible, but that would be a factor you did not mention. The populations on other colonies might be in trouble from conditions there, but that too would be a problem you did not mention.
- There is little to no awareness of the matter, nor any propaganda about making more children.
This isn't plausible because already humans have awareness and propaganda about the lower birth rate for more highly educated women, generally for economic/workforce/healthcare worries, but not for extinction worries.
With the increase in living standards, the number of children by woman is decreasing in highly developed countries.
I tend to think this has less to do with "living standards" and more to do with higher levels of education, and more choices for women in terms of professions, ability to support themselves, access to reproductive rights and services (birth control, abortion, marriage rights).
So, the issue you mention, which is that women tend to have fewer children when they have more education and choices, is something humans have been studying, but is in no danger of resulting in human population evaporating simply do to that trend, and certainly not without humans noticing it.
Most of the many actual threats to human survival have to do with the opposite problem, of increasing human population, leading to disasters from us destroying our planet's ability to support life through non-human habitat loss, non-human extinctions, climate change, pollution, WMD proliferation, pollution, clean water use, GMO pollution, nanotechnology, overfishing, ocean acidification, etc. We'd be better off with a declining population, and would have a much easier time surviving as a species and a planet if we lowered our population even to 1% of what it now is.
Industrial food consumption and other modern life conditions may lower fertility, and some future accident might make that situation far worse. But again, that's also not what you were asking about.
However, I suppose if we consider a future where humans overcome our current problems and survive long, long into the future and become very civilized in some form that has a situation where the fertile women are choosing not to have children very often, and perhaps that far-future civilization has become disinterested in maintaining its population levels, I could see it being a kind of issue, but I think attitudes towards reproduction and extinction would need to change, and/or some other factor such as greatly reduced fertility, before this cause would threaten humans with extinction.