AWS Backup vs AWS Data Lifecycle Management

Hello.

Back up EBS volumes for each instance.... How long will this take roughly? I know with DLM it can take several hours per snapshot

The time it takes to create a snapshot varies depending on writes, data size, etc.
This is true even if you use AWS Backup.
https://repost.aws/knowledge-center/ebs-snapshot-ec2-ami-creation-slow

Restore EBS volumes to existing instances. Essentially getting rid of old volumes

If the volume from which the snapshot is taken is the root volume, it is possible to restore the entire instance.

DLM is simply a feature that automates the creation of snapshots and AMIs.
AWS Backup also automatically creates AMIs and snapshots, but it is also possible to manage access to the created AMIs and snapshots, and protect them from deletion with a vault lock.