Personally, I'd go back to the battery, using wire suitable to the total current requirements and a fuse. Connect that to the C terminal of the relay. Connect the load to the NO terminal. If you have a number of light circuits, consider adding an auxiliary fuse box between the relay and the loads. That's what I did when I built my cab -- I have separate fuses for work lights, windshield wipers, fan and chute rotation motors.
I would only tap the tractor's wiring harness to supply switched power to the relay coil. The few hundred milliamps required to power the relay can be drawn from virtually any circuit controlled by the ignition switch.
When planning your lighting purchases, bear in mind that it takes exponentially more wattage to noticeably increase the intensity. In other words, it might take 4x the wattage to double the light output. Given that a tractor is a slow moving vehicle, I never found the need for long distance lighting. And LED units draw far less power per lumen than traditional bulbs. A 6 LED bar on either end of my cab seems to be more than adequate, provides far more light than the original equipment and is far less draw on my little 10 amp dynamo!
