All very true! Immich, unlike a lot of apps, does have a pretty friendly UI for managing/restoring backups, though: https://docs.immich.app/administration/backup-and-restore/

It's definitely still a lot more than the zero work required for Google Photos, but I feel like even even a semi-technical user can test an Immich backup without a ton of hassle. Start a new/empty immich instance in an empty directory, and follow the wizard for restoring a backup (and copy of your photos folder) taken from the non-empty instance.