The underlying similarity and logic of the exemption of zman grama for both women and slaves (and children) is clearly that "time is not their own" due to their historical subordinate status to master, father, husband, and related lack of autonomy in society in general and among Jews. However, this situation can be said not to apply halachically in our time. By way of example, the idea of eliminating mamzerut by having someone marry a "shifcha knaanit" is invalid because slavery is illegal in society.
It can be logically inferred that the fact that in society women have attained equal autonomy as legal persons means that the exemption for zman grama is inapplicable. AND this is not dependent on individual women "accepting the mitzvah" because it is an overall change in society. Therefore the implication is that women should recite the Shma with brachos twice daily and occasionally wear tefillin (since of course there is no halacha actually requiring that tefillin be worn at any specific frequency), and recite a tefilla twice a day (since the entire body of davening is not Talmudically required, and the maariv prayer is halachically optional) in less than 5 minutes.
Given the fact that acquiring human beings is prohibited in modern societies in any form, it can therefore be said that marriages by kinyan/ketubah are automatically INVALID just as acquiring a shifcha knaanit is invalid. Thus married women should NOT have to be subject to a get or agunahood at all nowadays! Marriages can thus be described simply as consensual partnerships carrying none of the effects of kinyan marriage.
David Goldman, USA
Firstly, I will refer to the rationales you stated for exempting women from time-based Mitzvos. The Avudraham (Sha'ar 3) and Kolbo (ch. 73) indeed state the reason you mention, that women are subordinate to their husbands and if they performed time-based Mitzvos it would cause marital strife. Based on this, Rav Chaim of Sanz maintained that women should not perform time-based Mitzvos even voluntarily and even in our time, as it could still cause disharmony. I believe even in our "emancipated" times, if both men and women wanted to go to Minyan with little children in the house it would cause some degree of disharmony. The other Mitzvos you mentioned would also cause havoc if performed by a woman daily.
However, all this is moot since there are reasons beyond our knowledge for the exemption of women. We can only surmise reasons of our own but we must respect the Torah's authority to exempt women in all time periods. Rav Moshe Feinstein zt'l emphasized this in a responsum (Orach Chayim 4:49) to feminists wishing to "change" the Halachah regarding this issue. He writes that even if life will totally be transformed and women will not take care of their kids anymore, the Torah's law is immutable and will never change regardless of the circumstances. (He does not say that they cannot perform these Mitzvos voluntarily if they have time, etc.)
As for your assertion that a man "acquires" a woman in marriage, this is a total misunderstanding of Kidushin, which includes a "form of acquisition" but by no means implies that a woman is acquired by her husband. She is betrothed and married but is no slave. This is not the meaning of the Torah nor was it ever the approach of Jews (as the Rambam delineates ten obligations of a man to his wife, and for slaves there are no such obligations). These obligations require a Kinyan so that they will be binding but not so that the woman will be "acquired." The Kesuvah is a bill of rights for a woman, not a bill of obligations. She can even say that she waives certain rights and certain obligations (Kesuvos 47) and has a large degree of autonomy in every marriage.
Yoel Domb
However the bottom line is that she is potentially subject to "release" exclusively by the husband through a get, exactly like an acquired slave. Otherwise she is always an eshes ish, meaning she belongs to the husband. And we know that acquiring a shifcha knaanis is not allowed anymore for terminating mamzerus because it is an invalid transaction in modern society. So there is the element here of actual acquisition like there is with a slave, and in both circumstances we can say that today a human status of "merchandise" is invalid.
Rav Hershel Shachter said that the classic Acharonim hold that the husband does not aquire anything monetarily. The Pnei Yehoshua says that there is a Kinyan Isur as opposed to a monetary Kinyan.
The Avnei Milu'im disagrees and says there is not even a Kinyan Isur, only a "Ma'aseh Kinyan" that brings about a "Chalos Ishus" -- a marriage. The means of creating the marriage requires a formal act of a Kinyan even though that act does not create a Kinyan.
Both agree that Kidushin is unrelated to a monetary Kinyan.
Kol Tuv,
Avraham Phillips