If you are comfortable with adding a couple steps, you could also link your notes within that folder. Then, you could filter your notes to that specific directory to see just the notes for that folder, effectively giving you a mind map in the graph view. A nice bonus is that if you have reference notes that aren’t linked, you see them just floating out there and know that you need to link them.
Just recently, the Obsidian team made it such that you can bookmark a graph view so you could save all your reference note mind maps as bookmarks.