There isn't a 1:1 app for Discord imo.
Selfhosting a teamspeak3 (ts3) server solves the voicechat.
Signal works great for text chats especially now that you don't need to give other end users your phone number.
Then I would probably look at hosting a web forum for adding calendars and other planning tools. There should still be possible to show current ts3 users on that site too.
For open source projects codeberg for code repository/issues/feedback.
I completely understand those who use Discord for ease of management, as time taken to host the above is time taken from the actual project.