Everyone should have a look at Hubzilla and its native ZOT protocol that supports nomadic identity. Open source, decrentralized, federated, all the right things. It is, in my opinion, a better system than the ActivityPub ecosystem. The closest surface comparison is probably Facebook, but it's much more powerful under the hood than that.
It's not like these tools do not exist yet, see: Hubzilla
Actually, there is a whole zoo of alternatives: The Fediverse, but I'd say that Hubzilla or Zap are the closest to what you describe.
Check out hubzilla. It's a fully decentralized, nomadic social network. You can self-host your own, or just start with their public instance. You create an account, then you create a "channel" for your group. All accounts are nomadic, so if you start with the public instance and then later want to go self-hosted, you can export your account and channel and re-import it into a new hub. It also can integrate with some other federated services like mastadon.
One of the first things you'll want to do (and any of your friends that try it as well) is go to "pin apps to navigation bar", go to events and click the pin. You'll get a calendar icon in the top right. Pin anything else that looks interesting. Click on the calendar to create an event and add it to your channel. Anyone subscribed to the channel will see the event, be able to comment on it, mark themself as attending, etc. If the channel's stream/feed is "too busy", all events are visible in the by going to the events app.
I admit that since the calendar isn't "pinned" by default and you have to go to install apps or pin apps link to find it, it's a bit awkward. I think if you self-host you can set some of this to show up by default, and you can do custom themes. But even though it's a bit non-intuitive to initially get to the calendar to add an event, once it's added, that event shows up just fine in the channel feed.
I tested hubzilla and was extremely impressed.
However, to limit your server costs, Is recommend running something lighter and simpler. I find people of all age groups hand bulletin board sites pretty well. To that end I'd recommend flaskbb.
It doesn't hurt to check out the selfhosted master list