I work at a small liberal arts college, St Michael's College in Vermont. I have a much greater expectation of teaching effort than professors at research institutions would have, although here there is also an expectation of professional activity. So the writing is not anything like full-time. But it is a creative outlet, which is great.

> I have a graduate degree in math, but I unfortunately I don't use math in my day job, so I am afraid that the material I produce will be too low in quality to be useful for people...

It is good to have feedback from real people. I work through material with my students in classes, and sometimes I've had what I thought was a great approach and it would just not work in practice.

I don't know if there are online groups of people working on writng stuff like this, either at the college level or at high school or earlier levels, but if you could find such a group, it'd be good to get in with them. (And tell me. :-) )