I kept a pomegranate growing for over 30 years treating it very much the way you are, outside for summer, under lights for winter.
I suggest, leave it outdoors into autumn. Allow it to get a few frosts. Pomegranate are winter hardy through about +25 F, (-4 C ) so let it let it get a few frosts. This should make it more or less fully deciduous. Don't worry if some leaves hang on. You want it to naturally drop about half its leaves. Then bring it indoors to run the rest of the winter under your lights. Warning, if it gets down to about 20 F (-7 C ) the pomegranate will be toast. The hard freeze will kill the roots.
Another suggestion. Don't prune in autumn. If necessary to prune to make it fit in your winter space, but don't prune off more than what is needed to allow it to fit. Most of your winter growth will be weak compared to outdoor, full sun, summer growth. Every spring I would prune for shape & style when I put it outside for the summer. Let it grow out, prune again a week or two after summer solstice. Last pruning for the year usually happens middle of august. Let it grow out through autumn. Only remove enough to get it to fit under lights when you bring it in. Prune if needed to keep it under lights.
Trick
To force a heavy cycle of blooming, quit fertilizing, except add 5ml to 7.5 ml by volume to 4 liters of water, (1 to 1.5 teaspoon per gallon) of epsom salts, magnesium sulfate. Water with this solution once a week for a month, then go back to regular fertilizer program. This should kick off a round of blooming. Best if done during middle of summer.
Magnesium sulfate can trigger blooming in a few species, if you have a flowering tree that blooms on new growth, pomegranate, roses, crape myrtle, bougainvillea, other summer blooming trees, magnesium sulfate can help initiate blooming.
This trick is not for species that bloom on previous years growth and require a dormancy prior to blooming. Here just let good growth and a cool dormancy set the timing for blooming.
