Hello! I'll answer both of your questions here if you're okay with that!

The main things to be aware of when nursing for longer periods of time is making sure you stay hydrated and eat a well and balanced meal. Breast milk contains a wide array of nutrients, but your body can replace these easily as long as you're hitting all of your food groups. If you have certain genetic dispositions to things such as, for a few examples, Vitamin D deficiency, parathyroid/thyroid deficiency, or pituitary insufficiency you very may well have to supplement hormones, vitamins, or minerals. Likewise if your diet is unbalanced due to allergies or sensitivities.

I, for example, eat a Ketogenic diet, which means I eat very few carbs in a day. Since breast milk is sugar, I have to make sure to eat enough fat for my milk to be the consistency I prefer. (Fat is simply sugar in long-term storage, which is why some women experience weight loss with lactation.) I also take a Calcium, Magnesium supplement to offset that I do not get a lot of these in my diet since things like cow's milk is high in carbs. I also take a women's vitamin daily. In all of my years of doing this and even doing a restrictive diet like the Ketogenic diet more recently, my blood work has come back clean each and every time. Just be diligent with your nutritional intake and supplement when necessary. Routine bloodwork, which can be ordered by your Primary Care Provider with a full metabolic panel can catch an insufficiency that you might be experiencing. Also, always be aware of symptoms like loss of motivation, apathy, and tiredness/weakness, as these are symptoms that could indicate you have slipped out of balance.

As for belly dancing, it would be like managing going out in public, and for you, since the bras are often rigid, it puts you at a bit of an advantage. Pump/hand express your milk before going to your show. If all your bras are rigid and not transparent, you can buy bra inserts/pads to soak up anything that might leak. If you are really worried, it seems like even the night-time nursing pads would be undetectable under a rigid bra.

Hope this helps!