I thought some people might find this of interest:
A year ago there was an NSF-funded project in the USA to develop a website to “curate” open educational resources for math. The website is here: https://curatedcourses.org/
Last summer I was invited to an AIM workshop that represented the tail end of the project. They didn’t accomplish everything they wanted (in particular, they only did linear algebra), but it’s an interesting idea: basically, they developed a tagging system for content (example: math.la.d.mat.rref for math -> linear algebra -> definition -> matrices -> reduced row-echelon form), and then applied that system to several textbooks, both commercial and non-commercial.
Along with the textbooks is a collection of other resources, like videos, tests, assignments, exercises, visualizations, etc., all of which are tagged accordingly. The idea is that once you choose your book, for each section you can easily access a large database of ancillary resources for your course.