I recently attended few events on Hopin and they were really good. But hopin is not an Open source platform.

Found this article while looking for os replacement for Hopin. Hope it helps.

https://www.reddit.com/r/opensource/comments/gldyc2/whats_an_alternative_to_hopinto_an_online_event/?utm_medium=android_app&utm_source=share

~Tawishi