I think your approach should work. It is unusual to use the Z coordinate after homing as an absolute Z position, but it should be stable. Typically people will touch off the workpiece and set zero that way, but your way should be more consistent for surfacing the spoilboard in multiple parts.
I don’t see anything wrong with your gcode either.
I suppose one thing you could do is home and measure the high spot multiple times, to confirm that the value is correct and your procedure is repeatable. Also make sure you lube the Z screws. When I first put my LR3 together and I hadn’t yet lubed the Z screws it appeared to be working and I didn’t realize it was missing steps and giving me uneven Z. (Although that would tend to produce the opposite problem, where it cuts too deep.)