I understand that one must blow softly on Bansuri, as if cooling a food item.
But what about how firm to hold Bansuri. I noticed my hands got tired after 6 minutes of playing because I held onto the Bansuri holes tightly.
As with any wind instrument you should only hold the instrument tightly enough to keep it stable, but no tighter. The problem with any flute type instrument is the balance and that takes some practice to get secure. Once you have the instrument balanced in your hands you need almost no finger pressure, just enough to keep the holes fully closed. Concentrate on precise finger movement rather than strength.
One cause of your hand tiredness might be the finger stretch, particularly if you are playing a larger instrument. You just need to start slowly and build up your endurance.