There are a lot of interesting opportunities before you. You may wish to get your younger daughter's input on the decisions, to see what she may be most comfortable with. Having a child's buy-in can have a wonderful positive effect on their desire to make it a good experience.

You may wish to look into the Iowa Acceleration Scale as a formal process with the school, possibly contemplating 2 grade skips...?

You may also wish to read up on advocacy... having a general plan in mind with the school may ultimately be much more successful than reacting to each opportunity in a piecemeal manner (which can become a nuisance, like swatting at mosquitos.)

Wish I had more definitive answers, but it truly is different for each child and learning environment.