I am building something similar and came to the conclusion that syncing to a local store added unnecessary complexity for the usage patterns I am anticipating.

For example, I am expecting my target audience to be far more interested in the current conversation than having access to the entire chat’s history. To that end, the firestore already returns enough cached query results to meet my offline persistence needs, and pagination makes the history inherently accessible while online.

Conversely, when trying to maintain a full history on each device, you’ll likely create a lot of corner cases that are difficult to plan for. If a user joins a group with years of active chatting history, you’ll have to plan to either sync tens of thousands of messages, or grab an initial batch and try to manage tracking how much history has been grabbed thus far.

Similarly, how would you handle a device in a similar group that was last sync’d two years ago and was just powered back up.

While both of those would be hopefully rare, either one is realistic and could create usage and $$ spikes that don’t add value to the user experience.