It seems like you are making promotions into something more like the reality- a recognition of a member's outstanding achievements and/or constant demonstration of competence. So... things are going to happen in battle, and the leadership (your commanding units) basically authorize promotion orders based on the outcome.
Perhaps as a part of the promotion, they are awarded better gear or a weapon of their choice (basically, when an upgraded class gains the ability to use a new weapon class) or are chosen for additional training (such as transitioning to fighting from horseback).
Also, I think you might find these to be helpful.
And this stuff I just think looks cool
I'd rp, if I wasn't bad at coming up with characters that weren't gary stus. Most every character usually has one glaring shortcoming and it's always like :feelsgn: and it's always really core, main characters that are well rounded enough to keep everyone together.
Like Lon'qu inability to deal with women. That one had me rolling my eyes.
Vapid Vaike, Ghost Kellam, shy dancer olivia, nightstalker tharja, insincere silver tongue Virion, stickler for rules knight, that kid who wants to grow up right now, the scooby snack assassin, Flo-bot mage, the sheltered noble, kirby knight (the one that eats everything), the one who lives at the gym (sully), the scrooge merchant, foreveryoungsyndrome...