Want to get this right from the start. We are going to be backing up to immutable cloud storage so don't want to make any mistakes from the beginning
Having done the reading beforehand it does look like an idea to separate out the jobs to Mail\Archive, Onedrive and then the Sharepoint\Teams data for performance.
What is the best way to achieve this? As far as i can see the only way to seperate out the Mail and Onedrive data is to choose users on the first screen and then edit those users for only mail\archive. But if that is done is that list dynamic for new users and the selction? My gut says no so leads me to the above question of how best to do that.
To have new users automatically added to processing, you can either build your backup job based on M365 groups, or add an entire organization to a job and adjust its processing options (i.e. create one job with organization/mail, another - with organization/OneDrive, etc.).