Hello,
I’ve faced a similar issue in my app. The solution was to customize the sync process. For the majority of the entities, use the sync Nothing(preserve data) and and avoid sign out (because all local storage is wiped on sign out).
Please, see https://docs.mendix.com/refguide/mobile/using-mobile-capabilities/offlinefirst-data/synchronization/#customizable-synchronization for details.
In addition, Marcel Groeneweg shared a feasible solution to handle large data in offline first mobile apps:
https://www.mendix.com/live/native-sync-for-large-data-sets/
Good luck!