In general, the trick to adding flavor complexity (ie, acidity) to a climacteric fruit is to pick it mature green and ripen indoors. This holds true even for jackfruit -- which if left to ripen on the tree retain zero acidity and are a little bland for my taste. But, it does take some trial and error, as one needs to be able to judge when the fruit is ready to be picked.
Most novices (including myself when I was new at this) believe that best flavor and sugar development is obtained when mangoes are allowed to ripen on tree. This is not true. Moreover, if fruit is prone to internal break down (ie, jelly seed), picking early mitigates the issue.
The fruit in the picture probably would have been good 2 - 3 days prior to when the picture was taken. I typically eat mine when the skin is totally green still. It takes some experimentation / trial and error to find the right stage. But once you find it, NDM is a top tier mango.
When allowed to fully ripen, NDM has a bit of a "nutty" flavor, which can be a bit offputting to the typical North American consumer.
I've already harvested about 1/2 of my NDM tree. Cutting the fruit a little early causes it to retain acidity.