Vitamins and/or lighting for indoor birds

[celtic_dragon]

I am raising three silkie chicks and they are doing great. They are now 4.5 weeks old. They will be primarily indoor birds and so I want to make sure that they get the right levels of nutrients and lighting to be healthy.

Right now they're getting Purina Start & Grow. I noticed that while this contains lots of other vitamins, there's no Vitamin D.
As treats they have been getting oats, cracked corn, and soldier fly larvae. I also bring clumps of grass, dirt, and other outdoor stuffs in for them each day to munch on and peck around in.

When the Start & Grow is used up, what should I feed them? I read that layer feed has enough Vitamin D, but that it is bad for roosters. Since they are silkies I won't know who is a rooster for quite some time.

I see that some people sprinkle regular Vit D pills in with the food or mix it in the water. Is that the best option (and if so, what's the correct quantity?)

Another option is to use a reptile UVB bulb. I use a Reptisun 10 for my bearded dragon but she also gets D3-calcium supplements on her food (see https://www.chewy.com/zoo-med-repti-calcium-d3-reptile/dp/124402) Would it be appropriate to use this same calcium/D3 supplement on the chickens' food? Or will that be too much calcium?

 

[lazy gardener]

Is there a reason why you won't be putting them outside?

 

[celtic_dragon]

I live in an apartment. I am committed to giving them enough space, exercise, love, and nutrition indoors. Once they are adult I plan to give them supervised outside time... otherwise, I bring the outside to them as much as possible. However I do not own a yard. I would really like an answer to my question. I know there are others who keep indoor chickens.

Edit: Does anyone have thoughts on this product?
Rooster Booster Mineral and Vitamin Pellets