Sorry for the late response. Yes you can use it completely offline. It's an offline-first app, you're in complete control of your data. You don't need to store it in the cloud.

Only if you need to sync with other devices then you can choose to do so by getting a subscription, but it's not a requirement to use the app at all. It only syncs your data to cloud if you're signed in and have a subscription, otherwise it doesn't.