For a simple graph app I don’t see why you’d need anything more than what you’re doing plus maybe some Maps of category names to nodes and a hash table of node ids to nodes for quick lookups.

It doesn’t sound like too much work to create your own in-memory cache - if you even need it...

Graph dbs are more for searching for patterns in your data. In your case it sounds more like you just want to traverse the graph and display a subset of it.