Worried About Pot Size + Root Damage Question

ok so my plant is 3 weeks old , an auto that will hopefully be ready to harvest by week 9-10.
I just transplanted her from a 1 gallon pot to a container that is at least 2 gallons and at most 3 gallons,
 I could see that the roots had reached the bottom and sides of the pot when I took her out of it,
that was after 3 weeks,
im wondering when does the most root growth take place in the plants life?
cos its seems that  it would drastically increase when the plants starts flowering because of the rapid growth above the soil; roots are for collecting nutrients and water, a bigger faster growing plant will need
more water and nutrients, right?
if this is the case I will need to transplant one more time I think unless you tell me otherwise

Also during the transplant it was little tricky getting her out of the pot but I made sure to be as gentle as possible, after loosening her up a bit I flipped her upside down and lifted the pot off, I looked inside the pot and seen that some  bits of roots (very few) had stuck to one side of the bottom and were ripped off when I lifted the pot . Does this matter? to me it seems it will be fine because of how small the damage was (like 4 or 5  2-1cm long bits of thin root) but I hear a lot of people saying that you shouldnt even touch your roots when your transplanting let alone physically damage them
what do you guys think?

 

thanks