I would like a sightseeing mode to look at the graphics.

[Harmonea]

Once a dungeon is completed, the Return skill takes you to the final boss room rather than the beginning of the dungeon

Have... you ever actually tried this, because it's completely inaccurate.

The shortcut at the start takes you to the final boss room. Return's functionality never changes, so it's never impossible to access earlier areas. And there aren't any current dungeons that have optional mobs. For older dungeons that do have such mobs, unsynced mode is exactly the "just exploring for graphics" mode that you want.