Are you using the built-in GFUI for scanning the certificate? If so, I’m not as familiar with that, and unsure how much adjustment you can do.
Alternatively, you could scan it using a regular scanner, and then do some image editing to get the image just right before importing it into GFUI.
I think you may want to make all of the text black in your image, so it engraves it at a consistent, higher power.
Right now it’s seeing the green as lighter than the black, and using less power (or some dot pattern or something which will appear lighter) when engraving.
Once you have a scan of the image, you can use GIMP, and the threshold effect: https://docs.gimp.org/en/gimp-tool-threshold.html
Or an online tool to perform the threshold affect: https://pinetools.com/threshold-image