When to start flower

Should roots be fully established ( circling entire pot) before flower or should I transplant right before flipping 12/12?

easy.....transplant
you don't want the plant to be root bound going into flower

You can throw her into flower any time after roots are established. I have flipped them mere minutes after transplanting a rooted clone into soil with mixed results. Best results I have are after I have given them a week or more after transplant to establish roots.

Let roots establish 1-2 weeks so there is good nute uptake for 12/12 stretch.

You can induce flowering whenever you want, brother. Some people start flowering during the first week. I would suggest letting your girls build a foundation first. At least 2-3 weeks Veg.

If your pots are too small now, transplant. If you still have decent room in the pot I wouldn't bother as long as it is a 3 gal or larger. The general rule of thumb is 1 gal per month of growth, so most plants require a 3 gal pot for 12 weeks of growth. If you do re-pot, I would wait a week before flipping them. Peace