Grbl M3 always uses M3 so dynamic power (M4) cannot be used even if Variable Laser Power is enabled, with same file, using Custom GCode with GRBL M3 and variable laser power enabled this is output:
;LightBurn Core 2.1.00-RC-6 @ 75b2b30 Qt6.5.7
;Custom GCode device profile, absolute coords
;Bounds: X20 Y20 to X70 Y20
G00 G17 G40
G21;Restore metric mode
G54
G90;Restore absolute mode
M3
;Cut @ 4999.998 mm/min, 50% power
M9
G0 X20Y20
;Layer Line
G1 X70S500F5000
;Cut @ 6000 mm/min, 60% power
M9
G0
M3
;Layer Line
G1 X20S600F6000
M9
M5
G90;Restore absolute mode
M2
I’im using Algolaser Delta, which has grblHAL based firmware and trying to use custom commands for Air Off and Air On which cannot be used on standard GRBL controller, and Custom GCode is missing constant power for it and also GCode clustering. Also GCode clustering cannot be used with Custom GCode.. Some info about that is in my old post https://forum.lightburnsoftware.com/t/looking-for-recommendations-for-a-new-laser-frame/132980/10?u=ztomic