I contribute to various open-source projects, so I have some experience with the GitHub ecosystem as a collaborator. I would love to help you, but I don't have the time or the experience to help you start a GitHub repo. However, I do want to pass on some words of encouragement.
Your plugins help out many people. In fact, I found this plugin after seeing another plugin author recommend yours as a better alternative. I understand and sympathize with the fact that you no longer want to keep developing this plugin. Maintaining a plugin for free is a time-consuming and exhausting task. Therefore, I think it would make the most sense to upload the source to GitHub so others can contribute and collaborate to make this plugin even greater.
Thank you for your hard work. Your plugin was very useful to me, and I wish you the best of luck, no matter whether you keep it closed-source or open-source.