The reason our range doesn't offer NRA courses is precisely that: time.
The NRA First Steps course or the Basic courses take 10-12 hours, a Cooper style Intro class takes four.
The four hour course is a money-maker, the NRA courses aren't.
Everyone has an agenda.
The range's agenda is to make money and get people to join. The NRA's agenda is political. The curriculums reflect the agenda, so I suggest you investigate where your support will come from (meaning who will provide the classroom, ammo and range) and their motives. Then choose which agenda you're comfortable participating in, because those who provide support will expect you to further their agenda.
Some ranges won't let you teach privately because you're competing with them, some will for a cut and some don't care. Investigate what your range's agenda is, what they will expect you to teach, (some ranges want you to give your students a guided tour of their gunshop, range benefits, and so on) and then, again, choose what you're comfortable participating in deliberately.
Which brings me to my next suggestion, don't try to do things under the table. Under the table is fine unless something goes wrong, or unless someone files a lawsuit.
Get the insurance, choose your support. Document what you teach and why you teach it that way. I'm not certain whether or not you can get the insurance unless you're certified. I was certified before I tried.
You're right, there's money to be made. In my experience, the Cooper Four style classes make more money and sell better than the NRA classes. And I don't feel a need to give a lecture segment on "Why People Own Guns". (Part of the NRA curriculum.) So I teach the Cooper Four style almost exclusively, unless someone asks for an NRA course.
And good luck.