There is not definitive answer to most of your questions, I'm afraid. A lot of this varies from person to person, and the only way to know is trial and error.
Some people will say xyz formula works for "most people" or "HANDS DOWN" should work for everyone. I believe they are wrong.
Personally, for gut health, especially in regards to things like dysbiosis, I believe it's better to avoid starches and sugars. I haven't had problems with starchy veg like carrots or sweet potatoes, but regular potatoes, white rice, and even too much fruit can disrupt my gut bacteria (basing this on the available evidence, from yeast infections). I can usually eat a couple of pieces of fruit a day without issue, however. Refined sugars are COMPLETELY triggering for me. Even too much maple syrup (more than 1 tbsp in a serving).
I cannot answer your questions on serotonin either, though I've wondered about those. My gut health is poor and the worse it gets, the more my mental health suffers. I believe it's linked as a psychological and physiological response. Ratios and amounts don't do jack for me right now.