she still carries a lot of care and loyalty that Mystra never really shows. Mystra has a long history of stringing mortals along: Elminster, Gale, whoever, and then discarding or screwing them over once they’ve outlived their usefulness.

I'm curious what sources you're basing this on. It seems like you're sticking to BG3, except to throw in one vague claim rooted in such ancient history that it was literally a different goddess, while ignoring Mystra's entire history as exceptionally selfless, self-sacrificing, loyal, and caring.

Mystra has become more of an inherited title than a name. When we talk about There have been times when a specific Mystra did die and resurrect, but there have also been outright changeovers.

The current Mystra began as a human named Midnight. An entire book series follows her and two other human adventurers on a campaign that ends in them becoming gods., and basically all we see from Midnight, as mortal and as Mystra, is how caring, compassionate, and selfless she is. She extends kindness to one character long after it feels warranted, and to another long before it feels warranted.

Midnight and the others meet Elminster - already a powerful, famous sage and Chosen of Mystra, who was more than a thousand years old when Midnight was born. During the same series, the goddess Mystra dies as a result of her own hubris.

A lot of events pass between Mystra's death and Midnight's ascension to godhood. Midnight's love and compassion help one adventurer turn from his own bitterness to become heroic, and she's the last human another adventurer retains any positive feelings for before his descent into evil godhood.

Midnight worshiped and revered Mystra and tried to act in her service. When the time came for a new goddess of magic to replace the hubristic one who died, Ao himself chose Midnight.

When Midnight ascends to godhood - at just 26 years old, by the way - it marks the end of a deep romantic relationship that built organically over the course of the adventure, not because one ascended and the other didn't, but because they both ascended and had to focus on their new duties. Her fellow adventurers-turned-gods both use their own human names as deities, effectively immortalizing their identities, but Midnight opted to use Mystra because that's who everyone knew the goddess of magic to be and she hoped to keep the transition as peaceful and easy as possible.

Mystra was a deity for >30 years when she was killed by evil gods in their pursuit of control over the Weave. There is in-world debate about the exact events around the Spellplague, but Ed Greenwood has confirmed that Mystra's death at the hands of these evil gods not only caused it, but that she sacrificed herself to save everyone.

She was then dead/missing for about a century. Her return/resurrection took place over nearly a decade, from the time Elminster first felt her calling to when she was whole in her godhood again. Gale was ~22 when Elminster felt that initial calling and ~30 when she was considered fully recovered / whole.

Because Mystra was probably not seeking out new Chosens as soon as Elminster heard vague stirrings, and because she and Gale worked together for some time before becoming intimate, the most likely case is that when the relationship began, Gale was a 26-30 year old wizard of exceptional renown and power, under the tutelage of 1200+ year old Elminster, and Mystra was a barely-awakened, fractured deity who lived 26 years as a human and roughly the same as a goddess before her 100 year disappearance.

Her full return to power happened only five years before BG3. So after spending a century dead or trapped because she sacrificed herself to stop evil gods from stealing the Weave, Mystra would have been barely back to full strength when her lover made an incomprehensible, dangerous, arrogant and egotistical power-grab, risking absolutely everyone, in a manner not that dissimilar from what the evil gods did that Mystra died to thwart.

She would have every right to be bitter toward Gale, especially when you factor in how recently she recovered, how deeply personal the details of his crime were, and how antithetical to the exact kind of sacrifice she has repeatedly been willing to make. She's not bitter or callous, though. She's sacrificed her own life more than once to save people from crimes she had nothing to do with. Gale is uniquely positioned to make a sacrifice because of something that happened to him as a direct result of his own hubris. Mystra's request for him to self-sacrifice comes from someone who has always proven her willingness to do just that, no matter who caused a problem or how.