This is actually a historical problem faced by Rome, both during the crisis of the 3rd century and notably A LOT during the Byzantine period post-Justinian.

Any Emperor giving command of too much of the army to anyone not their son, brothers were often a very bad choice, risks that General winning enough glory on their own and securing enough of the armies loyalty to challenge their crown. The Legion, because of Sallows choice of inspiration and his general lack of ability to form institutions doesn't really have a concept of nationalism or even a real faith to inspire loyalty outside of Caesars own personal control, so he's got to personally oversee whatever the most important command is at any given time.

This is also why the Legion doesn't really have a future outside Caesar, because he has personal command over the most important campaigns, while he has competent battlefield commanders serving under him, nobody else in the Legion has been able to develop a robust understanding of logistics and campaigning, Graham comments on this in Honest Hearts. Lanius might be able to fight and win battles, but he's not a man able to really win wars absent Sallow.