The concern about portions of the Military having nothing to do is valid, although this has been taken into consideration early on and is an essential part of the Empires hierarchy. The ranks of the Empire do not necessarily qualify certain roles in the Empire, they are primarily there to demonstrate who is in command of who, and anyone with a higher level of power has complete control of subordinates to terminate them from the military to promote them to any level that is not higher or equal to their own. A captain of the Empire now, for example, can choose his own squad commanders at will, de-rank and terminate members. A squad commander cannot though promote a soldier to the squad commander position or above but can terminate them from the Military. (Although appealing is easy and the majority of the time soldiers terminated will just be reassigned to another squad unless the person who terminated you has provided a strong case with evidence)
The roles system, on the other hand, is completely separate from the ranking system, consider it as a certification system. All soldiers will have the base Infantry role which they are trained for, but to be a pilot it requires additional training before you are given that certification and allowed to fly in the Military, other certifications such as Grenadier or Enforcer will require small courses on the Imperial capital lasting up to an hour before you are able to serve that role in the Empire. Some certificates can only be granted on the other hand, such as Elite, Royal Guard or Advisor. Imagine the ranking system is the base template for power and order and the role system is the specialisations you can optionally receive.
In the field and by launch squads will be better suited to both timezones and roles, so there could be a full squad of grenadiers to operate artillery in the field and a full squad of pilots. It makes everything easier to organise for battle commanders.
In terms of boredom and being out of duty there's a tonne of roles for soldiers to optionally take on when not in a battle that will severely help the Empire, for example when in deep space (Travel in some instances could take weeks in the game by launch) These soldiers, if they have an interest, can work in maintenance to ensure the Destroyer is operational for the journey and ready for battle, a programmer can work in the R&D labs during these periods to assist with the next mission, or Hanger managers and Traffic controllers can monitor the patrols of squads during defensive periods, by launch there is hope to be potentially hundreds of roles covering any level of interest.
Now, of course, I can estimate the next question will be about maintaining all these permissions, changes and roles. How such a fluid system is kept in check and managed without constant communication and people to keep track of all the changes. This is also something that has been in consideration for a very long time and is currently still under heavy development. It's called the Imperial network and it's a piece of software that any imperial can log into to see a live ranking chart, apply for roles, request meetings or even view the next mission and destination you need to be at. It will be a mostly automated micro-management system that will support the Military. Ordinance requests can be sent and you will have a profile that lists your squad, any direct messages, events and emergency alerts. We don't want this system to feel like a boring portal though, it will be designed intuitively to feel like it's part of the game and isn't running the military but just available to support you if you are lost within orders or changes. Within a glance, you should know what is happening on the field and where you should be. I do urge you not to ask the specifics on how the data will be input into the network and how certain tasks will be delegated, just know there is a team within the Empire working countless hours on this network and it's been under development for over a year and have considered every question at this point.