you are incorrect, the DTV is not just for attending a course in this case. Having a booked course is just one of various pathways to obtaining a DTV. By its very nature it’s a tourist visa. Once you complete your course (or never even attended as attendance is also not a requirement) you are free to spend the rest of your five years traveling around Thailand on tourist status border hopping every 180 days. There is no ongoing requirement to attend any course, you are exercising your right as a tourist.
What you are referring to on the DTV visa is a free text line. This free text line is Embassy policy or even IO specific. Some embassies leave it blank, some write “Employment Prohibited “, some write the soft power activity you applied for, some write “Workation”, it’s a free text line, anything could be written there.
There is no follow up by Immigration. You merely have to qualify during the application process. If you extend in country you would again have to have a qualifying course booked. If you border bounce currently there are no checks.