The essential thing to do is to vacuum package the mushrooms before deep freezing. Only vacuum packaged deep frozen mushrooms keep their colour and texture. The vacuum package machine has to have a strong pump and to be at least of semi-professional quality. I can't recommend a product here because I haven't found out what the machine I use (Solis Genius) is called outside Switzerland. It could be "Foodsaver" but I am not sure.
Vacuum packed deep frozen mushrooms keep at least a year. We had 3 year old deep frozen mushrooms and they were as fresh as on the day they were picked.