Thinking out loud. With a nice library around the Feedbin API it wouldn’t be too hard to grab the data and stuff it into SQLite. Alternatively, a Feedbin account could be registered with NetNewsWire and then the underlying SQLite DB inspected.
The former seems more elegant while the latter is radically pragmatic.
If for nothing else, poking around in the NetNewsWire SQLite DB probably illustrates a highly performant SQL data model and schemas for RSS data and feed management. Or even better, just read the source code