Miles turns 20 in January 2023. Enrolling early doesn't matter unless doing so advances her graduation date and makes her eligible for the WNBA draft before she's 22 years old.
A bit confusing the way you phrased it initially ("if she sticks around another 2 years after she becomes draft eligible"). I don't see a scenario in which Miles will be in a position to forego 2 years of eligibility at ND unless she is eligible for the 2023 draft. She won't be eligible for the 2023 draft, at 20 years of age, unless she's ready to graduate in May 2023. I doubt that's the case.
Assuming she's not draft eligible in 2023, she'll be eligible for the 2024 draft but only if she's ready to graduate in May of 2024 (she'll be less than 22 years old). If draft eligible in 2024, she'll only be foregoing 1 year of eligibility at ND.