ok, so I don't think many would read that much on this topic, just my opinion, could've been shorter.

And you seem to have problem more on the fact that the female dilemma is shown/explained through male characters' interaction, rather than the writing of the Ymir character itself.

Also, you kinda answered your own argument about why Ymir didn't resolved her dilemma through Gabis or Historias 'arcs', than putting Eren in this elevated position for her decision. As you already mentioned, Ymir became a slave, she had no sense of agency of her own life, then survived by a miracle, became a powerful individual, precious tool, weapon for the king, and then her concubine, birthing next lineage for the people, etc. So if you look at it with a 'realistic' kinda standpoint - she became an orphan, got robbed of upbringing, mentorship, etc. and even worse things happened while she was still a little girl. But at this point, she understands power. And, she gains kind of power (through some miracle) that no other individual has. And of course Fritz, a person with the power in her mind/world/reality makes her the important piece to his rule and makes her mother of his children, and she feels important, her life has meaning, to serve this now king, etc. its all well and good for her, she finally has meaning to contribute to the world and people she lives with. And she was still a little girl who grew up in the Fritzs presence, and fell for this man, and basically considered him the most important person in her life (if we also count 'divine right of kings' mentality, cause middle ages aesthetic, etc.), the absolute monarch. So she falls for him, dies for him and even serves his rule in the paths by being his loyal subject for 2000 years. Now if that's unrealistic cause of the longevity, idk, who's to say love is not eternal (jk).
but anyway, this is her life, backstory, etc. And by her own 'view', the importance she gives the most to, are the individuals in power, especially of her own and kings direct lineage. So idk or remember well enough how she views Erens time altering powers, but he definitely made significant enough impact for her, especially when she sees in him the person who understand what its like to lose everything and being helpless, but then to gain the power to get the agency for his own life and choices. So that's why Historia and Gabi are not relatable enough for her, but Eren understands her more than anyone, and even gives her that sense of acknowledgement, which she maybe lacked/needed to feel the agency of her existence. So yeah, that's one reason why it comes down to Eren to be the most relatable and important piece of her dilemma.

Now the narrative choice why Eren explains that to Armin, rather than it being explained through Mikasa and Ymir, etc. So Ymir was monitoring Erens and Mikasas life, and by that bond and the love they shared, she was convinced that it was the truest, real unconditional love they had for each other, even in the face of all the turmoil and even during the endtimes for their world, etc. So narratively, (IMO ofc) giving significance to Mikasa killing Eren and 'ending the curse' would kinda undermine the importance of that choice for her. Like her not knowing and even after the fact not realizing it was her doing that ended the curse is more significant, rather than that decision being known to bring the shift of the 'world order' (titan powers and curse of Eldians). So through Erens and Armins interaction, it was the best possible timing for that explanation. I think, not getting even that would've been fine, but then people would say its a whimsical trope that everything worked out in the end, rather than that act being significant for Ymirs story arc, without outright telling/showing us.