The onboard air is convenient. I’d just run one line for both bags though and don’t mess with the self leveling. Been there, done that. It was a total headache and ended up trashing all the parts. Those systems are just way too finicky. Total waste of time and money. You can read my build thread to see the details. I currently have onboard air and one line to each bag. Sometimes one side is higher than the other for some reason, so I have to make adjustments. If there is just one line, the pressure will equalize between the bags. Now, I know that a full tank of gas, which is on the drivers side, can make a difference, but it’s minimal. No need for different springs. If you are hauling more than what those airbags can handle, you’re overloading your truck. Plus, when unloaded, you’re most likely going have a pretty harsh ride.