Bubbles in the crust have a lot to do with the fermentation.
Over-fermenting causes the yeast to run out of food and then the gasses stop being produced.
The heat does expand the bubbles, but they have to be there in the first place.
Under-fermenting gives more bubbles, but then you run the risk of the center of the pizza bubbling.
It you want a more crispy crust, try baking at a lower temp for longer time.
If you want bubbles, ferment for shorter time or gradually increase yeast.
Only make one change at a time. If you do too many, you don't know what made it worse or better.
It is a long process. Don't give up...