Effects of "Backup frequency" for continuous backups?

That really depends on the nature of the workload on the instance I would say. Every time the snapshot is taken ( after first full backup) , it only records the blocks that have changed. Therefore, if the workload is highly dynamic, there are chances that the blocks are changed every time the hourly backup is performed, resulting into high storage consumption and hence quickly add up to the cost specially if the snapshots are retained for longer time. But if the workload is static in nature, there might not be any changes during the hourly backups and therefore, less storage consumption and cost. So, it all depends on the nature of the data and desired RPO.

For continuous backups, the schedule in AWS Backup defines how often a job is run to

1. Check the backup is running as it should be, and start it if it isn't;
2. Check for retention points that have expired and need to be deleted.

The underlying process for continuously backing up data will not be affected by the schedule unless it's not already running, and there is no impact on the storage consumption as all changes are captured. If regular snapshots were being taken instead, multiple changes between those snapshots would not be captured.

If you look at a continuous backup in a vault, you will see the recovery point ID has a date from when it was first created. When you restore from it you can choose any point in time up to the most recent update.