Pruning 1st year Japanese Maple seedlings

Plymouth, MA

This winter, I had great luck stratifying hand-picked Japanese Maple seeds and by March I had 64 germinated seeds after having started the process in November. Fast forward 10 weeks and all 64 have come along nicely under my grow lights for 18 hrs a day.
Now that the seedlings are well-established (5-8 leaf pairs per plant), I am wondering if anyone has recommendations for next steps. Given the qty, I have plenty of room to experiment with them and would like to force 50% into multi-trunk growth and let the remainder follow whatever course they happen to.
Is now an appropriate time to prune the tops? Or should I wait till fall? Or next spring before leaves have opened?
Where on the trees should I make the cut? On the crown? Above the first leaf pair? Somewhere in between?
Aside from grooming timing/technique, I am also curious about how I might accelerate growth over the few first years. For example, by pulling the trees inside mid winter and starting the growing season earlier under artificial light, or maybe even going so far as to try to fit in two growth seasons per year by including an artificial winter (ie 2 x 4-month grow seasons separated by 2 x 2-month winters).
Anyone have experience on any of these topics? Just looking to experiment, learn, have fun, and care for plants!

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