While I don’t have the 40W to test, theoretically since it has the focusing arm, use that for your focal length. Home the machine, note the machine coord of Z, then move the focus arm down, and jog down until it touches the top of the laser plate. Note the machine coord of Z again. Take the homing height, subtract the focus height to get the current position above bed focus. (i.e. If at home, it says machine Z330, subtract focus Z23 to get Z307)
Remove the lines;
G53
G0 Z23 F6000
and the subsequent G0 Z300. Change the G92 Z0 line to G92 Z307 substituting for your result instead. This tells the machine it’s 307mm above the 0 origin without having to move there. Then you can use the material height setting in Lightburn for auto focusing.
As far as the M8/M9, it’s a known issue over at Lightburn. Since the Snapmaker never had air assist before, it kind of ignores that setting. Hopefully should be rectified in the next release. However, if you swap to using the Marlin device profile, it should output M8, at least it does for me on the latest 14.0.3 version of Lightburn.