Roger,
Hate to tell you… but none of this stuff is “a piece of cake” 
Here is a lengthy thread from a year or two ago, that will give you some history behind the firmware mods (that I think you’ve applied) that had to be applied to get good engravings from the later versions of Marlin. In that same thread, I also discovered Victor’s ImageToGcode program, which seemed to give me better results at the time than the Image2Gcode program I had been using… and which you are using now. It’s confusing but, though similarly named, these are NOT the same program.
TBH I don’t remember much about all the testing and discussion we did (probably an age thing…I’d have to re-read the thread) and don’t know whether those mods actually made it into the current Marlin build (Ryan and Jeff should know…). Since then I’ve come to prefer Grbl for my stable of machines (I don’t currently have any set up with Marlin…) and discovered Lightburn. Grbl just seems better suited for laser use with its “laser mode” (M4 power adjustment with feedrate), ubiquity, and ease of use. And Lightburn runs beautifully in Linux and is very powerful/versatile while both Image2Gcode and ImageToGcode are Windows programs… so Lightburn has become my preference and I’m happy to pay the $40 for a license.
In short, all I can suggest is to read through the referenced threads and hope that someone who is currently and successfully using a laser with Marlin will speak up. I’ll also suggest that it’s hard to beat chipboard (cereal-box cardboard) when testing your laser setup… it’s cheaper and more uniform/consistent than real wood, which varies greatly from piece to piece, even of the same species. Finally, I must admit I NEVER got the results I’m seeing now (with Grbl/Lightburn) with any of my Marlin-based machines. Maybe sufficient progress has been made in Marlin that it’s now possible to get the same results… just saying, I never saw it when I was actively using it.
Sorry I can’t be of more help. Good luck!
– David