Converting GeoTIFF to LAS/LAZ is as easy as this thanks to pdal:

pdal translate in.tif out.laz --reader gdal --readers.gdal.header="z"

if it is a one band raster you want to convert. For others you will have to adjust the readers.gdal.header part, e.g. to --readers.gdal.header="Red, Green, Blue". See https://pdal.io/stages/readers.gdal.html.