All in one solution, your describing nextcloud. Though if you started in signal maybe its not the right solution for what you’re doing? if the group is official enough for there to be donation and tax records maybe go nextcloud.

https://github.com/stoatchat/ is an option, its made good progress since discords first try at age restrictions.

setting up a few matrix servers is probably the more secure way to go. and either mumble for voice or jitsi if you want voice and video chat capabilities. decentralize and avoid a situation where one self hoster leaving or one domain seizure shuts everything down.