When you use Direct to Object backups you don’t have the option for Synthetic - it is only Active Fulls that you can select within the job settings.
To answer the question of “least resource consuming backup type”, the answer is object storage. There’s no concept of full or incrementals. Assuming this is a virtual machine or Veeam Agent-based backup, and not a file share backup itself (aka NAS Backup).
Instead, your backup is split into small objects, you define the pre-data reduction block size during the backup job, and uploaded in these individual objects. Metadata is used to stitch the objects together to create your backup points that you need from these. This means that if 5% of your blocks change, you upload those 5% new blocks, and when blocks become unneeded by any surviving backups, they are deleted.
The block size I mentioned is defined here: https://helpcenter.veeam.com/docs/backup/vsphere/backup_job_advanced_storage_vm.html?ver=120
Veeam factors this into immutability by increasing the age of each object to the minimum immutability period of the latest backup in which they are required.
@Chris.Childerhose @MicoolPaul
Thank you very much for your pertinent answers.
If I use a backup copy, is it compatible with the full synthetic configured on the initial job?
Absolutely, Veeam handles the converting of the media between block-based backups and object-based backup copies, so it doesn’t matter whether you use reverse/forward/forever-forward and any active/synthetic fulls. Veeam can handle this transparently for you.