An alternative might be Coursera or UCLA Extension possibly?

One online alternative might actually be the Chinese University of Hong Kong which is surprisingly lax for gifted students and accepts grade skips more readily.
(https://www.math.cuhk.edu.hk/e-learning/online-e-learning-center#:~:text=The%20online%20E%2DLearning%20Center,MATH4230%20%2D%20Optimization%20Theory)