I'm not familiar with the course but it seems to be well regarded. I can't tell if it's truly free or not but if it is, you could give it a try.
It looks like the levels are from beginning to intermediate. Intermediate in this case is still grade school level I think.
A well-regarded textbook series you could look into is the
Genki series. It's comprehensive and often used in universities.
And many people use it for self-study as well.