DD is more than 3SD above the mean so they know it's coming at some stage.
That's an interesting notion. DD4's new school recommends subject acceleration as needed but they are very cautious about grade skipping for many reasons.
I can see why this is a difficult decision. My natural inclination would be to leave her where she is if she is happy but you don't want to wait until she starts getting miserable. When my DD4 became bored at her last school, oh my, did she let her teacher know it. Talk about being challenging when not being challenged. I definitely do not recommend waiting until the proverbial has hit the fan.
Her current teacher sounds great and seems to understand your DD's needs. What does she recommend?