Women having more than 1 child is not necessarily result of their biological urge, but also other pressures and influences, as well as, in some cases, lack of access to contraception or autonomy over the decision of how many children to have, and at what age.

The reasons education in women results in having fewer children may more likely include that:

A) they understand that having more children is linked to a considerable financial strain / increases likelihood of poverty for the family, and make choices accordingly.

B) they simply start having children later, which shortens the biological window during which child rearing would occur; and also delays the first attempt at conception to potentially past biological prime (ie it takes more time of trying to have the first child, further reducing the overal "maximum attainable child rearing potential" so to speak). This may also be result of likely being matched with older partners who also may be past their biological peak.

C) they are more autonomous in decisions on how many children to have, and when to have them, and more aware of other avenues for self-actualisation that will be shut or considerably reduced for them by having more children. (Not necessarily career pursuits. Even some affluent women who are home makers choose to have one child.)

D) If a girl child is born to a family that wishes for the girl to be educated and enables this for her (which will sound odd to people in societies and communities where you don't need enablement from parents to study), the parents may overall have other life outcomes for her in mind than getting married and rearing children, and may be further influencing her in that direction even after she completes her studies. And the same would be true for influence from educated peers.

D) The fertility crisis overall (declining rates in both men and women, some of it due to exposure to toxicity).

I would agree that women being educated probably raises their expectations on their prospective life partners overall (where they have control over that choice), but that may be about many factors other than or additional to the partner's educational attainment... e.g. their inner maturity, lack of self-centered behaviours, and so on, which may shrink their prospective marriage pools considerably, for reasons outside of their influence.

I don't think the measured correlation between educational attainment in women and reduced birth rates is specific to places where women have higher average attainment than men (which is not all places) - or is it? (Haven't checked stats).

I agree that declining birth rates have significant consequences for a population collapse, and - in the context of this subreddit - especially for what constitutes a functional exctinction treshold (learnt a new term just yesterday! ;) ) for humanity.

In a scenario a few decades later, when humanity's population may have realistically already declined dramatically, and fertility may have continued declining with accumulating toxicity and compounding exposure to it... (and with other health factors reducing fertility and birth rates potentially increasing)... the burden of reaching the 2.1 maintenance level average birth rate would be placed disproportionately on a subset of women or couples who are still able to conceive. I.e. certain women would be nominally required to have (many?) more than 2 children in order to compensate. And this may be incompatible with their personal aspirations and choices, or even physical ability. (Which to me does open prospects of scary Handmaind' Tale scenarios).

Also, if this is past societal collapse, which it realistically could be, rates of women dying in child birth may go up to pre-modern medicine and surgery levels, which would further reduce chances of women able to conceive having many children in order to hit the average 2.1 rate.

All of this suggests that the functional extinction threshold may be much higher than most of us would intuitively think, and some might even posit we are past the threshold.

What I don't agree with is OP's reasoning for why educational attainment in women results in reduced birth rates, which appears only applicable to some contexts and also looks away from a range of other at least equally plausible explanations.