A few things I would change.
There is no need for the fuse after the Relay You are not going to gain any current going through the relay, Just the one from the Battery will suffice.
I would move the switch to in between the side light and the relay save you running the wiring back from the lights up to the switch you can Earth it to the frame closer to the lights.
Also the reason you use a relay is so the high current for things like lights don't burn out the contact in the switch, unless you were to use a large switch of course.
As for adding a circuit for high beam you could simply use the existing relay with a line from the high beam but add a diode to both the activation lines to the relay to stop either circuit activating the existing lights.
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