What I did:
You say: Download Agenda for iOS app for free from the iOS App Store and sync your notes across all your devices, Mac, iPhone and iPad alike. All you need to do is to make sure you’re logged in to the same iCloud account on all your devices. Like Agenda for macOS, the iOS app has no time limits.
Can Family Share be used to sync Agenda notes so that the same iCloud account would not be required? I am unaware of a way to stay logged in to two iCloud accounts (one for Agenda sync and one for unshared data) at the same time except in Family Share.