Is it possible to host a private, defederated, and completely isolated instance for use as your own social media platform?

I want to set up a collaborative writing/socializing platform for my friend group using something like Calckey/Misskey, and just want to know if this is possible.

The non Fediverse options all look very lacking, and are meant for corporate/business environments anyway. It really feels like there's not many good and modern options for this sort of thing, but maybe there's better alternatives. Who knows, I'm really new to this scene.