Aside from cost, there are several key considerations that should govern your decision when selecting the right instance size for your needs:

Network Speed:

If backing up from premise to the cloud, the network speed of your AWS instance should move your backup data to the cloud fast enough to meet your backup window requirements.

RAM:

SoftNAS uses the RAM of the AWS instance for caching.  Instances with more RAM can increase the performance of your backup jobs.

Read Cache:

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® 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