I'm thinking about creating a brilliant.org clone (the old version, not the current enshittified one) but entirely community-driven and open source: Besides the classical features (create/delete,edit your own problems) you can also have courses and collections: courses are a ordered sequence of lessons and problems, lessons are post-like entities that can contain text, images, animations, embedded videos through links, latex, code, ... Collections are just a ordered sequence of problems (ideally it is a progressively difficult path of problems, but this feature can be used for anything you want). It would also feature a wiki, and eventually a forum for discussing/administering the website.

If anyone wants to join the project, contact me replying at this comment/writing at gbc0 [at] proton [dot] me