Your not supposed to use romex as a cord. You should use rubber cord rated for weather. If you can I would run a weatherproof panel to where you plan on having the breaker,then off of that put an outlet. You can then plug the lights into that. Code wise you need to have the breaker box mounted,can't use rx as a cord and it would be exposed using it like that. What you said would work but not the safest thing to do with rx. If you did decide to do it that way then maybe put a post in the ground to hold the breaker panel up and still needs to be weatherproof. Anyway to run new outlets from panel to close to the location of your lights, then you could put the lights on a timer by panel. Outlets outside would be gfi so protects the lights from ground faults
