Caching - custom development or package from pub.dev?

So I'm running a pretty basic set up. Firebase / Firestore for a lot of the backend.

I usually did a bunch of read writes but I'm anticipating a large jump on user count soon (it's an internal tool for a business that's about to expand), and while I'm not paying for usage obviously the client won't be too happy about seeing a bill were we could get rid of one (for now) with some more optimisations.

So I'm trying to get all relevant user side information stored inside of a JSON file stored locally and then read from it whenever I need to get user data, this includes objects related to the user depending on their location so it's not as straightforward as just doing custom claims (although that's already done).

So it's a custom job right now, but it's going sluggish. The caching packages I've seen are not really what I'm after.

Basically we'd be storing a number of lists of objects like location data, user data and some physical retail item data that I don't want to have to retrieve each time the user say does a stocktake.

Any ideas on a cache package that I'm not seeing? Something that is more than just caching from HTML requests etc, or stick to custom development?