As far as I can find out, FaceTime is currently the only solution for "just works" (in the sense that your grandma could use it) videoconferencing that is e2e encrypted.
If there are no cross-platform apps which support e2e-encrypted multi-party videoconferences, then there simply is currently no solution at all for 'just works' secure videoconferencing, because part of just working is not requiring users to switch to a platform they otherwise would not.
