Why were you not able to transfer it? Connect the GPS to your computer and open the Garmin folder. If necessary, create a folder named CustomMaps. Copy the Google Earth .kmz file to that folder.
You must use .jpg images for the .kmz, it will not work with .png images. Make sure that the image you created is no larger than 1024 x 1024 pixels. Also, only use letters and numbers in the filename. There's an old bug where a file named my_map.kmz would not work but mymap.kmz does work. This may have been fixed though, not sure.
I assume you already know where to find the LIDAR imagery, since you're already using it? If it has 1-meter resolution, then each map tile can be about 1km x 1km (1024m x 1024m). You can use 100 of these tiles at the same time.
For more versatility, you would download the actual DEM files from the USGS, then use some software to render them in whatever style you like. Sounds like MojaveMan has some ideas for this. I use GlobalMapper, which makes all of this trivial, and it can directly create the Garmin custom maps. It is expensive software however.
Again, look at the other thread i linked to:
https://forums.gpsfiledepot.com/index.php?topic=4556.0As discussed there, qGIS and mapc2mapc are useful tools for this. qGIS is free, open source software Mapc2mapc is not free however. g-raster is also mentioned in that thread, and is another tool that is worth a look. Again, it is not free.
https://moagu.com/?page_id=155