Tasks.org offers several ways to combine local tasks with synchronization. Choose the app distribution and sync provider deliberately, because the free F-Droid feature scope differs from Google Play subscriptions and the separate Tasks.org Cloud service.
Choose the distribution and synchronization method first so the cost expectations are clear. Start with a few lists and use subtasks only when actions have a real hierarchy. Test reminders and synchronization on a small list before moving a large task collection into the app.