Course platforms end at checkout. The finishing, the questions and the second purchase all happen somewhere else — so put the course where the people already are.
From first sale to finished
The bit after checkout is the bit that decides your refund rate.
Questions get answered by someone
A student stuck at 3am posts in the feed and another student answers. The thread stays for the next person who gets stuck in the same place.
Threaded discussion one level deep, so long threads stay readable
Link and video embeds, so people can point at exactly what they mean
Reactions earn points, which surfaces the students worth listening to