Thanks for the info, that does help me understand what's going on in the protocol.

I suppose I was thinking about the app responsible for getting the data into, say, Leaflet's local database in the first place. For that app, the data structures you describe are the APIs that it consumes, for the sole purpose of populating its own database.

That gets my cogs whirring about how good a use-case this would be for SQLite. I suppose it would be somewhat appropriate for a small, private, PDS? Although, if I understand correctly, I wouldn't need to worry about that anyway if I'm using some off-the shelf PDS software, e.g. https://atproto.com/guides/self-hosting