Depends how you define self hosted but I use Easy Gmail Scheduler (https://github.com/RayBB/easy-gmail-scheduler https://github.com/RayBB/easy-gmail-scheduler).

Depends how you define self hosted but I use Easy Gmail Scheduler (https://github.com/RayBB/easy-gmail-scheduler https://github.com/RayBB/easy-gmail-scheduler). To send gmail messages later without any third party. Disclosure: I made the app.