There is an excellent YouTube playlist from Laser Everything covering all aspects of Galvo and Lightburn, I would urge you to look at it, especially how to calibrate the lens.
To start with, go through the following video to sort out your timings.
Will of course try this! But could this really be what causes problems only in certain places? Feels strange in the middle of a row, for example, like on the flag.
If you look at the file in preview, you’ll see that the section mid flack is distinct, and the galvo head will still be moving; if it’s not turning off exactly at that point you will get the overburn/shadow.