awful toxic abuelita character

I cannot stand her, she's unbearable. I hate his whole family, they're horrible

This is much more complicated though than that Elena simply was a jerk.
Yes, it is really a heart-breaking scene when she destroys the guitar.
And it's clear that even Miguel's father who otherwise agreed that becoming a musician would be bad for his son thought that it was too much.
But that is how much Imelda poisoned Elena's mind with her fear that any Rivera who became a musician would disappear from the family.
Which is clearly so unacceptable in Mexico, that Miguel had to learn a lesson about the importance of family as if he had done something wrong.
And as I come from Sweden, I know that this makes little to no sense from a modern Occidental perspective.
But from a Latin American perspective, what Elena did would be seen as harsh but understandable when you consider the circumstances.
And as weird as this sounds, Elena destroying the guitar was also a necessary step in Miguel's journey to save Hector from the final death.
Because if Miguel still had had his old guitar, he wouldn't have taken Hector's guitar in Ernesto's mausoleum and gone to the land of the dead.
Which he just had to do to learn the truth and be able to refresh Coco's memory, so she could talk about her father before she passed away.
So if you look at the moment when Elena destroyed the guitar within the context of Mexican culture in general and this story in particular?
Then it's clear that Miguel had every right to be upset and angry, but he shouldn't have said that he didn't want to be a part of his family.
And it was like Elena had to destroy his guitar, so he could save Hector and make the Riveras heal and stop being dysfunctional at last.
And that is the end of this wall of text, but it truly is a much more complex story than that Elena simply was a jerk for no good reason...

Yeah, she was easy to resent, but it's not a cut-and-dried case of villainy.

One the one hand, the "No Music" rule was clearly someone making a stupid rule because they love being obeyed.

On the other hand, they're aren't a rich family, everyone has to work and pull their weight in the family business, there's no room in that house for a kid who thinks making shoes is beneath him.

And as for her never apologizing, that seems to be realistic for a story about Latin America.
Lots of people have expressed disbelief in that Abuela Alma in "Encanto" apologized.