How to keep frozen beans soft?
I think some fats may help better than just sugar? I have never tried this but it is worth the experimentation. If you add heavy cream (dairy) it may help. Coconut cream is high in fat and could also help in keeping it soft at lower temperatures. It is the ice crystals that make it hard, so the key is to separate the ice crystals at the microscopic level. Fat droplets can help in that if they are mixed well. 

If not, just work with some scentless cooking oil in small quantities. As @Ecnerwal mentioned, read through the ingredients of the store bought ice cream and see what other ingredients are present that you may not have in your recipe. Based on some mung bean recipes online, it appears that egg  yolks also help with the texture. Good luck!