Well it sounds like the game doesn't really offer what you are looking for. There isn't really any "difficult light party dungeons" outside the later parts of the two deep dungeons. Hardcore content is mainly limited to 8 person content at endgame. And about 80% of the game at least is solo content, particularly the MSQ. The game is a Final Fantasy first, MMO second, so the main story takes up a large chunk of the content. And outside dungeons and trials it is not multiplayer.

Hope you find something more your speed.