Would self-hosting a Nextcloud instance locally without an internet connection be viable?
Use case: Around 5 people need to share files over the network, collaborate on Office documents in real-time, use GitLab, and a To-do/Task management tool.
Beyond the initial setup, does any of these requirements need an active Internet connection, or can we all connect to the Raspberry Pi server via Ethernet?
It is not very difficult with a router OS like OpenWRT or OPNsense, but I guess it might still try to do DNS queries, not 100% sure. You could probably also run your own local DNS server, but I never tried that.
What you can maybe do is add a second mobile connection as a fallback via Mwan3 if that is a bit more reliable.
But maybe something like Syncthing and desktop apps is in the end more reliable then stuff that is really meant for internet facing services.