Last year I got myself off sweeteners by buying better coffees for a cafetière rather than the instant I was drinking. Now I love all types of black coffee and couldn’t imagine adding anything to it. If your coffee is bitter and hard to drink could you try different coffees black ?
Ultimately it’s more a personal preference than physiological one, everyone has different goals and bodies. For folks that are fasting for specific digestive health or religious reasons, probably going to avoid anything that isn’t water. For those fasting for general wellness, the usual black coffee, tea, etc. For weight loss, there’s an argument that anything that gets you to your next meal without eating is going to be worth it.
I can’t answer your question, but I can relate to the desire of adding something to my coffee. I decided to wait with my coffee until my fast is over. I look forward to it every day and see it as a treat now to get my coffee. I drink tea and water during my fast… Good luck!