I'd suggest you start with the manager role and remove all permissions related to editing courses or grading.
An easy way to figure out which permissions you need would be to use our Moodle sandbox demo and log in as an admin in one browser and as a manager in another browser. You can then use the admin account to edit the manager role and use the manager account to test the changes. (You just need to refresh the page for any permissions changes to be shown for the manager account.)
The Moodle sandbox demo sites resets every hour on the hour, so a good plan is to start just after the hourly reset to give yourself plenty of time.
Once you are happy with your customised manager role, you can export it from the Moodle sandbox demo site and then import it in your own Moodle site.