Use a self-hosted Git service. For example, with Gitea, you can gather repository mirrors using multiple GitHub tokens (which you can later keep up to date, similar to GitHub forks).

• What is Gitea? - Gitea Docs
• Repository Mirror - Gitea Docs
• Mirroring a GitLab or GitHub repository to a self-hosted Gitea server - StackOverflow

With Gitea, you can either publish the packages using their API and configure HTTP access as well.

• Configuring the package registry - Gitea Docs