From your description I’m not sure you understand Mastodon. People who don’t know about it and read your description will get the wrong impression, at least.
You can follow someone on any Mastodon server from your account on any server. It’s not siloed like Discord (which, ironically, actually is a centralized service!) - it’s much more like Twitter than Discord in real use.
There may be no official ‘core experience’, but there is a de-facto ‘core experience’: a stream of posts from people you follow, from any server, in chronological order.
You can follow someone on any Mastodon server from your account on any server. It’s not siloed like Discord (which, ironically, actually is a centralized service!) - it’s much more like Twitter than Discord in real use.
There may be no official ‘core experience’, but there is a de-facto ‘core experience’: a stream of posts from people you follow, from any server, in chronological order.