SQL Data Masking a copy of data for backup

I need to send a database backup to a vendor for an upgrade, and somehow need to mask several columns containing PII. Was looking into static data masking, but this seems to change the data permanently. Dynamic data masking seems better as it hides it as it is accessed by users without unmask permissions. But once this database is backed up and sent off for the vendor to restore, could the vendor not grant themselves higher level permissions to see the data anyway? They will likely need db_owner anyway to make all the structural changes needed for the upgrade. Is there a better (hopefully low-effort) way to mask certain columns without permanently altering data for a backup?