First, welcome! My thoughts as the parent of a moderately gifted (MG) child are that you will absolutely need the services of a private gifted school. Our public school was able to accommodate our MG child in K last year through subject acceleration, and hopefully will this year too, but we've had some frustrations. I cannot imagine how our public school or our district's HGT (highly gifted and talented) magnet program would accommodate a PG child like yours.
The HGT programs in our area are not designed for PG kids. They're designed for kids who are working about two, maybe three, grade levels ahead. Your DD16-months is working at kindergarten level, at least four years ahead of her age. Based on everything I've read, no, your DD isn't likely to level out; I agree with mumofthree's assessment above.
The Davidson Young Scholars program would be of use to you in the future. I know very little about it, and hope others on this site will give you some information on the program.
You may know this already, but in case not - when preparing financially for private school, only a small percent of the money in a 529 plan counts against you for financial aid. A higher percent of the money in an UTMA counts against you. And of course retirement plans are not counted, unlike taxable accounts. Where you put your money can make a big difference in your ability to get financial aid in the future.
