I don't think it's that niche, Sight Reading Factory (the site, iOS app, etc.) has been around for years and is pretty popular as a means of learning sight read

I don't think it's that niche, Sight Reading Factory (the site, iOS app, etc.) has been around for years and is pretty popular as a means of learning sight reading for various instruments. https://www.sightreadingfactory.com https://www.sightreadingfactory.com