the amount of flour you use for a sponge is up to you and the flavor profile you are after. Typically 60%-90%, and I have seen as little as 25% of the total flour. You might want to test some small batches first to compare. Here is some related information: https://bakerpedia.com/processes/sponge-and-dough/
Ascorbic acid can be used as a dough oxidizing agent. It does not matter if organic or not. Obtaining organic wheat flour might represent more of a challenge in your region. Ascorbic can vary fro 30 to 60 ppm, and typical GMO is 90 ppm. More is not better. It is not harmful, and over oxidizing the dough will produce poor results. https://bakerpedia.com/ingredients/ascorbic-acid/
Vinegar is sometimes added to lower the dough pH and act as a preservative of sorts to slow mold growth. This will depend on the type and concentration you use, and the desired dough pH you want to try to achieve.
Ginger and thyme are strong flavour spices, and I would start at 0.1% and work your way up to a desired flavor profile. It should net be dominant in my mind, and more of a hint, like a fragrance. Fennel and coconut milk are not as strong, and you could start at 1%. Of course you will also have to adjust for moisture.
I hope this gives you some information to work with.