My guess is that we won’t be doing any digging on asteroids at first. We will send a Spaceship (with 100tn of cargo capacity) and grab a whole small asteroid. Then the spaceship with the small asteroid Inside the cargo bay will return to earth. It sounds science fiction but don’t forget we have already collected material from asteroids twice (the Japanese returned with 5gr and tre US 250gr).
You don’t even have to go very far, there are hundreds of small asteroids between the moon and earth. I think we will be able to collect a whole small asteroid in 10 years max.