Assuming your other bases are covered (like child care), your proposal for partial homeschooling sounds solid. As you said, it would be hard for her to learn less than in the alternative.
In the meantime, I'd continue to monitor the situation at the elementary school, because connections not withstanding, it's entirely possible that the new GT teacher is not there, or is not in the same capacity, to start the new year.
I'd also start planning to make the jump early, because your oldest DD in particular might start behaving in ways that indicate a need for urgency. Been there.
