Is it permissible to leave the employment of a merchant, after gaining experience and learning all the secrets of the trade, and then start working independently in the same field, knowing that I have the financial means to do so? And is this considered a betrayal of the employer?

If you cease working for a merchant, there is no harm in you engaging in any type of trade, even if it is the same as your previous work. This is not considered cheating or betrayal, for a person is free to pursue lawful earnings after concluding their employment. However, before that, you must abide by his advice and not engage in any private trade that exploits his name or business.