Better get used to that concern about "finishing too quickly!" Programs designed around age-typical development (aka, most schools) generally do struggle with a fear of running out of material, and of placing children with much older cognitive peers. Multi-age settings sometimes can be more flexible.
I agree that you should cross the K bridge when it becomes more imminent; it may be that your school will (as they did this time) approach you first because her readiness for K will be undeniable. That would be the easiest solution. There are also other options besides whole-grade acceleration, so keep in mind that getting "stuck" in preK and moving early to K are not the only possibilities.
And I wouldn't be so sure that she is not actually reading!
