Happiness and tax rate are very easy to manage and understand how they work.
You only need gold for researches and the salaries of your heroes.
So it goes like this. Before doing a research, while you are scouting cities to plunder of gold, set your tax rate to 100% so you will get like 10k-20k-30k or more / hour. The idea is to get as much gold as possible because it won’t matter if you get attacked and lose all your gold. You might need an extra amount of gold after a plunder and these 30k generated by your city will be handy.
After you have your gold for your research, and you start it, set the tax rate to zero as you don’t need gold until the next research. If you set the tax rate to 20% for example and you generate gold, you only give free gold to the attackers as you don’t have what to do with it until the next research.
Don’t try to defend your gold as you will only lose your troops, there is always someone with a higher might value than you.