From several other experiments, I choosed 3 developers that worked very well, using minimal amounts of substances.
I liked very much the idea of Reinhold of making a chart of his caffenols, so here is another chart searching for simplicity:

I made an experiment of mixing equal parts of the developers Black Caffenol (name suggested by Jay De Fehr) and Orange Pure. If coffee and Vit C aren't additive the time of development should remain the same. And it remained the same. Only making the developer twice as concentrated the time starts declining.
The Caffenol C21 is basically a 5% solution of sodium carbonate with minimal amounts of coffee and Vit. C in the proportion 2/1. It gives very balanced negatives, whereas Orange Pure gives no blacks and Caffenol Black gives plenty.
Interesting is that keeping the ammounts of stuffs but for just 1 liter water, we find a similarity to the fine developers proposed by Reinhold, with shorter developing times.