Help with pruning

So this here is just one blue mystic fem at 3 weeks from seed... my first grow ever. Should I do any pruning at this point yet? She is very bushy, and the overall growth has slowed as the shaded undergrowth is trying to catch up. Should I leave this undergrowth alone?? I want overall size to be just shy of half the PC tower she is in. I've read that 24 light versus 18/6 during veg will make them more bushy. Is that why shes no low and bushed-out?? Should I or can I switch to 18/6 to finish the next 3 to 4 weeks before going 12/12?? Will that cause issues and would that even help with the super bushy-ness I'm getting???

Not sure if the pics even help, but it sure is bushy under there. I'm planning to start the flower cycle at the end of Oct.

Thank you

I leave my vegetation to take clones from before flowering. I wouldn't touch it.

given the limited height u have I would probably go right to 12/12. the longer u veg the harder it is going to be to keep that plant from out growing your space