tape backup job which takes a full backup directly to the tape, without using any disk space.
You can achieve this only by setting a short retention on disk job (for example one day retention)
How does this differ from the schedule of the job itself? I also noticed that the schedule for creating synthetic full backups as part of the tape job does not allow me to specify a time, only days
Synthetic full backup to tape is controlled by tape job. Yes, you set the exact date in the Schedule, but it will use the existing backup chain, specifically it will pick the increment from the disk and merged it with the full backup and then transfer it to tape.
Job schedule starts the tape job, then synthetic full backup schedule is checked and the increment for the specified day is transformed in to full. If there are multiple increments on the set day, the last will be used. Do not forget that you need to enable incremental backup on the tape job (proses increments option) – otherwise synthetic full will be skipped (this behavior will be corrected in the upcoming patch 2)
The last but not the least please make sure you are up to date: Veeam Backup & Replication 8.0 Patch 1