Aside from cost, there are several key considerations that should govern your decision when selecting the right instance size for your needs:
...
SoftNAS also recommends the use of a disk Read Cache for backup jobs that use features such as Synthetic Full Backups, such as Veeam backups. SoftNAS Cloud® SoftNAS® Essentials does not allow the creation of EBS Block storage devices (S3 Object only), so you should consider instances that have an attached ephemeral SSD device which can be configured as a high-speed Read Cache. SoftNAS recommends a Read Cache of at least 10% the size of your standard backup job.
Recommendations
Alternate Recommendations for Large Jobs