after retention is reached the oldest incremental will be merged into the full backup.
In your scenario we have to merge 500 GB. This could take time. What do you use
as backup repository?
If you would accept that every backup is a Full Backup you can use this command to create a standalone backup via
script. "C:\Program Files\Veeam\Endpoint Backup\Veeam.EndPoint.Manager.exe" /standalone"
This avoid the need of merging backup files.
I'm not sure if this will work with Backup 2 Tape.
System Engineer @ Veeam Software
NetApp NCIE-SAN for 7-Mode and Clustered Ontap