I typically use User Origin as I don’t have a precise reference to position things on the laser bed. I use the laser on low power to set the user origin on the touchscreen when running a job from there. The position of the job origin is job dependent, I wouldn’t say I always put it in the bottom left. Sometimes it’s centered. Sometimes it’s middle right. Depends on what I’m doing.
Personally, I use a script where I can right click on the gcode file and send it to the touchscreen without using Luban: File Transfer via WiFi - #13 by brent113
More frequently I’ll run jobs directly from Lightburn as it can run gcode faster and the specific jobs I run are limited by the touchscreen’s speeds. This does not apply to you unless you’re doing grayscale mode with at least a .1mm spatial resolution.
Not even sure what this is, I don’t use Luban. I’d guess you’re referring to the part of Luban involved in generating gcode which wouldn’t apply here since the gcode has already been generated by Lightburn.