For a chocolate brownie mix, because I would want to make a moist kind of a brownie, I would use a mixture of baking powder with soy yoghurt. If you don't have yoghurt, then I'd use baking powder and oil (preferably coconut).
I have forever been skeptical of the flax egg and I recently tried it, I made quinoa kale bytes and it worked quite well. I would say ground flax is your best bet but consider the moisture you are losing so add a little apple sauce.
I've used the chickpea water (or any bean water for that matter) also known as aquafaba, as an egg replacer and it works really well but at the end of the day all an egg is doing is acting as a binding agent so anything that does that will usually work! I don't use pre-made mixes myself, so can only really comment on what works in standard recipes though.