I’m not sure if deleting the account on github would be the best option. They could use it as mirror and link to the forgejo instance where active development takes place.

I’d go for Matrix, with Element or Schildichat (not the next gen versions for now) as clients. It’s federated, which means you can choose which instance you want your account to reside on, end to end encrypted, has (video) calls, and some other features you wouldn’t necessarily expect from others like spaces for groups with several group chats, prioritizing conversations (low, normal, high / favorite) and threads inside a conversation.

See here for hosts:

https://nextcloud.com/partners/