I actually think a Mobilizon instance might be useful. Yes, the UI isn't the most customizable, but it does federate and people with Mobilizon accounts can interact pretty deeply with any Mobilizon group you make.
You could create a group there (you need a Mobilizon account to manage and administer the group, but Mastodon/etc can follow the "group account" (not your Mobilizon one) to get posts and events sent to their timeline.