I am interested in this. Do you have a website / would you mind posting your contact info?
I think there is absolutely a niche: students (or graduates) who have some experience with undergraduate-level math courses but that need to improve their fundamentals. Most US universities give credit for "Advanced Placement" courses taken in high school, even though these courses are rarely taught to university standards. I skipped the intro calculus sequence at my school, and since the proof-based math courses are usually fairly self-contained, I never got a chance to address the gaps in my fundamentals. I've now found myself in this somewhat awkward position where I can e.g. explain to you what sequential compactness means but can't solve for the roots of a third-degree polynomial.
It doesn't make sense to read a high-school level textbook, and books on competition math are either too hard or the focus is more on exposing you to hard problems rather than teaching fundamentals. There needs to be something which reviews fundamentals in a rigorous way.