You may be misunderstanding what a Full backup is in the context of a File backup. The only data that will be backed up is the data in the folders selected. Periodically, you'll have a run scheduled full backups - at least once a month. But, again, those backups are only going to back up the selected folders in the plan and the Full backup will also be a synthetic backup - which only runs an incremental backup and then uses the data in S3 to construct the new Full backup in the cloud.

What are your retention needs? And what GFS settings do you need (Weekly, Monthly, Annual). That will help me understand what your settings for Keep Backups For and GFS should be and what your scheduling will have to look like. Immutability is currently tied to GFS, and GFS settings will dictate how frequently you need to run full backups.

Thanks for the info - yes, I didn't understand that a full backup still only backed up the directories I selected.

I would like to backup all files in specific directories to immutable storage and have those files kept for a certain number of days. Let's say 30 days as an example. I see options for Weekly, Monthly, and Yearly, but not daily, so I guess I'd have to keep only the most recent 7 days, plus weekly, monthly, and yearly backups?

The way immutable storage works is:
1 - You need to use the new backup format
2 - You need to enable GFS as Immutability is currently tied to that option
3 - You need to schedule full backups on your preferred schedule - at least once a month
4- The full backups will be synthetic on supported cloud platforms and will only back up new data. The cloud APIs will be used to create the next full in the cloud from data that is mostly already stored there. The new full backup will contain a new generational copy of all files
5- You can schedule incremental backups to run on schedule in-between the full backups
6- Set your Keep Backups For option to 30 days
7- Set GFS options to coordinate with your full backup schedule - so if you're using an every 30 days' full backup schedule, then all you need to keep is 1 Monthly backup in GFS
8 - Enable Immutability - you need to create the bucket new and enable immutability at creation time. You cannot use an existing bucket if it was created without immutability
8- You'll end up with 2 full backup sets in storage before the oldest once can be removed

Unless you change the full backup schedule, there is no way to only keep 7 days as you're going to have a backup chain that includes a full + 30 incremental backups - there is no way to remove any of that backup data unless you schedule the full backup at least once a week.