You're right - it's great to be able to crosspost, but every fediverse software is totally allowed their own features and if other systems don't display it correctly, that's also ok.
But in this case, Mastodon really is missing some kind of "group" feature. We kind of have this with the a.gup.pe - Tool, but it would be nice to bring some order into that chaos.
You might try it again - I had quite the experience reading some of my old school lectures again without having to discuss this in class or write tests about them.