I'd probably steer literacy a bit, myself.
Math is easy enough to differentiate, in our experience, since it is decoupled rather well from everything ELSE in the curriculum.
Literacy skills
are a lot of the curriculum otherwise in K through 2, however, and in years 3 and 4, there is a marked transition from "learning to read" into "reading to learn."
I'd be quite concerned that for your DD, "learning to read" is
all that she is going to be getting out of her first several years in school, and
she may see little POINT in doing so-- if you leave it to the school to do so. Why? well, because you noted that she already knows so much of the material that she's going to be expected to read-to-learn, even, as a 3rd-4th grader.
So in my mind, there IS only one "gap" here that even matters much from a functional standpoint-- literacy skills.
They might entertain acceleration without her knowing what the difference is between a "town" and a "neighborhood" or that plant cells have cell walls and animal cells don't... but not reading fluently is a way to deny acceleration across the board. It
might even be a way to deny acceleration in mathematics, given that break from learning-to-read vs. reading-to-learn, and the fact that she's beyond it in math.
So I'd (gently) offer a variety of games/activities that encourage phonemic awareness, offer up some Letter People and Between the Lions on Youtube or the PBS website, etc. Letter hopscotch, rhyming games, you name it-- it's all part of the plan.
Once she learns to read for herself... she can take that tool and learn whatever she fancies with it, YK? That is
definitely not a "schooly" kind of thing-- but it will also provide her with some
relief in the event that she gets stuck in a classroom where she ISN'T learning anything new, too-- at least (with arrangements) she can probably unobtrusively
read as enrichment, anyway.
