Can I start assembly without having the X Controller?
There would be no reason to not start. All the wiring can be done post assembly.
The Xcontroller is literally the last thing you need to do.
So yes, do all mechanical assembly, take your time to square everything and tighten everything then when you get the Xcontroller, just wire it up and you’ll be good to go!
Emphasis on this. You will regret it if not everthing is squared. Have a look here