I have a free group that's just for people who bought my book and some other courses.
I don't do anything to nurture the group except for popping in to answer questions when I can.
When the ability to sell a one-time course came out, I added a paid course to my Skool group. It got some sales.
Then I spoke with and he showed me how to do something painfully obvious. (I didn't realize we could do this.) I implemented it in about 30 seconds and it made around $1,000 last week with zero promotion.
This video shows what I did.
TLDR: Free Group + Paid Course = Awesome. Yay!