Chromosomal issues are not the only factors that can cause an increased NT. Heart defects are another common, but less discussed reason for high NT. The routine anatomy scan is NOT an effective test for even complex congenital heart defects (only detects CHD prenatally around 25% of the time).

Regardless of whether you spring for the amnio, I would recommend asking about a fetal echocardiogram to get a better idea of heart function. Some heart defects need immediate treatment at birth so finding out prenatally can often be important for getting appropriate care. The society for maternal fetal medicine also recommends fetal echocardiography with increased NT.

https://www.smfm.org/publications/116-when-a-normal-karyotype-accompanies-increased-nt