Once you're a king, the recruits should come to you, rather than you having to go from village to village to collect a few men, just like how now you don't have to manually collect tax from villages.
1. Build encampment, this act as the gathering point. Castles will do also. This affects how many recruits can be trained and leveled up in parallel. At the moment skills like raise the meek increase exp in a pool which is the same as adding exp for only one person in a stack.
2. Have recruits come to your camp, the rate is based on your renounce and influence, or order companions to go to the villages to spread the words, and/or companions collect the recruits for you (though, this kind of can be done already by forming parties and let them run around for a while and then form army to gather them back). Recruiting troops as a king or a famous arena champion would feel very different than when you're still a nobody having to go from village to village yourself begging for recruit.
3. Train fresh recruit in your camp up one or two tiers, the leveling cost can be much higher than leveling from battles as now you're providing them the equipment yourself. It's quite off putting when literally your men can't carry shields unless they first go gut some looters.
4. From then on the soldiers will have to be blooded, by fighting in real battles to become veteran.
5. Different cultures can have different leveling caps from training for different units, or even unique additional ways of conducting training, imagine Vladian can train horsemen one tier higher/train faster than other cultures by conducting jousts. Aserai allow using prisoners as training dummies for large exp boost, or turn prisoners into Mamluk soldiers. Sturgians apart from normal training also allow pitting your own men against each other in duels, kill/wound/demote the other for instance level up to veteran. Battanians have the training rate increased based on the number of veteran/nobles units in your party. Imperial is more streamlined in training and can train infantry one tier higher, look at the current troop tree it's already Recruit-> Infantryman -> Trained infantryman. Finally, Khuzaits no training camp, can only level up by fighting real battles but can sacrifice prisoners to increase exp rate from battles. I think it would also give different feels and more depth to different cultures.
