Your journal is one file in your Drive, named marginalia-journal.json. This app can open only files it created, nothing else in your Drive.
When you create the Google sign-in for this app, give Google this exact redirect address:
Paste it here on each device, or put it in config.js once.
Download a copy of everything as a file you can keep anywhere.
Bring in a backup file, or paste the backup you copied from the earlier version. Existing entries are kept.