The colonial governments had previously had influence over laws in that their elected officials would advise the governors. England shut that down. Patrick Henry made the “taxation without representation” argument ten years before the Declaration of Independence
Many excuses were made, that’s the one that stuck. The real reason was that the colonies were rich, and now had effectively unlimited land to the west they could expand into, that the British army had taken care of the French.
I think you have that backwards. The grievances were the motivation. The situation was the opportunity.
If they were content, they wouldn’t have revolted
They weren’t content because they were now rich and the motherland was now poor. It wasn’t some high-minded ideal, it was opportunism.