I like how you explained this. I agree that slang isn’t lazy. I think it can actually be really creative, especially the kind young people come up with, like you mentioned with your son. I think it’s smart that you want to sound more natural in English by using colloquial phrases, while also knowing when to avoid them in formal situations.

I also agree with you about non-native speakers. This is very important. It really depends on their level. Sometimes slang makes conversations friendlier, but other times it’s best to keep it simple so the meaning is clear.
Notes:
Your writing is very clear and easy to follow, no major errors and nive wide vocabulary use. Just a few small points where it could sound a bit smoother in English:

“One reason why I have signed up for this course”
→ More natural as: “One reason I signed up for this course.”

“Of course, my main aim is to be understood, to get the message through.”
→ Could be a little smoother: “Of course, my main aim is to be understood and to get my message across.”