I find that doing multiple laps while dropping the Z axis a wee bit tween, helps get it done. It also helps to cut on a grid or such, so the laser can cut through. Lightburn has some other options for cut through, where the path hangs a little at the start and end of each cut, so the laser has just a wee bit more time to dig a bit deeper. Have not played with the settings enough to determine the real value.
This is 3mm Birch that I ran at 250mm/m@100% with 7 passes and a half mm of Z drop each pass.
Cheers
Anjin