Anecdote time: [Hi from Argentina!] During 2020 everyone and their dog made a similar project with Moodle (not as nice, but easier to implement in a few weeks). At the peak, I had to use at work 4 Moodles and my wife 4 Moodles, and only 2 were in common. A friend has like 5 or 6. Note that all of them were in the same University! And all of them had a slightly diferent configuration so transfering quiz from one to another was sometimes a problem.
I participated actively building questions for one of them before the pandemic, and we copied a lot of the stuff to one or two of the others.
I don't know about selling this stuff, but I think it will be difficult.
Anyway, I have some feedback about the questions. I tried the free ones. In the calculus example, when the computer replaces
something + 7x + c
with x=-4, it uses
something + 7 × - 4 + c
We usualy prefer that the students use parenthesis arround the negative values like
something + 7 × (- 4) + c
In a midterm we will put a warning, but not substract points if they don't use the parenthesis. Most of the times the students don't write the × and the equation is confusing.
Also in the next step
something + - 28 + c
I strongly preffer
something + (- 28) + c