Women Obligating Themselves to Observe Mitzvot

See Mishnah Sh'qlaim 1:5 below
This Mishnah AISI is a paradigm for 3 categories
1. Those obligated - Yisroel and Levi
2. Those who are NOT obligated but MAY contribute
- women, children, and perhaps Kohanim
3. Those whose contributions are rejected - EG Kuthim
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Seems pashut to me
"Asher Kid'shanu" applies as an obligation on class #1
And as an OPTION for class #2.
AIUI when category #2 performs, they take on the minutiae of the mitzvah
EG a woman need not take arba minim, but if she DOES take them - then, she may not do so at night nor using a lemon instead of an Etrog Etc.
Hence the b'rachah makes sense in that she performs the mitzvah K'dat and K'din
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Tefilas n'davah is another one. One need not daven it, but once one does, he has to conform to the rules of the amidah. He cannot just say EG r'fo'einu as the only middle 'brachah.
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This is how I understand the Ran [a Sephardi] as quoted by Bet Yosef [EG Hilchot Tzitzit, Sukkah, etc.]

While the Rambam paskens that women may absolutely NOT say a b'rachah, the BY differs in that he concludes that it's a s'feiq b'racha l'haqeil, and is omitted out of DOUBT, not out of certainty. Yes he seems to pasken the same omission, but the dynamic underlying it is different
The Rema following Ran says we definitely say the b'rachah; the permission to do this is ratified by both Kaf haChaim and Ben Ish Chai who state that a firm Minhag trumps s'feiq b'rachah

Kein Nireh Li

Source Text
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מסכת שקלים פרק א
א,ה אף על פי שאמרו, אין ממשכנין נשים ועבדים וקטנים; אבל אם שקלו, מקבלין מידם. הנוכרי והכותי ששקלו, אין מקבלין מידם; אין מקבלין מידם קיני זבים, קיני זבות, קיני יולדות. חטאות ואשמות, מקבלין מידם. זה הכלל--כל שהוא נידר ונידב, מקבלין מידם; וכל שאינו לא נידר ולא נידב, אין מקבלין מידם. וכן הוא מפורש על ידי עזרא, "לא לכם ולנו, לבנות בית לאלוהינו" (עזרא ד,ג).

Shalom and Regards, RRW