feliik cannot "back up to internal storage, then copying the .SeedVault directory to a USB flash drive" since
- There is no option to backup to internal storage.
What happens when you tap "Backup location" in SeedVault? Is there not an option with the name of your device? That represents the device's internal storage.
- I don't have a .SeedVault directory.
Backing up to internal storage is what will create the .SeedVault directory, so fixing #1 is required for #2.