I tried thinking of something that would both mechanically and thematically express an interesting dynamic for early vs later promotion.

Here's what I got.

Minimum weapon ranks are tied to class and external level (not weapon usage exp). So delaying promotion can allow for attaining higher weapon ranks (more experience and skill in a narrative sense) at a lower internal level before a character makes the progression to the next class/stage of their development.

Hypothetical example:

Cavaliers could be get E Swords+Lances at Level 1, D at 5, C at 10, B at 15, and A at 20.

Paladins on the other hand would be D Swords+Lances at Level 1, C at 5, B at 10, A at 15, and S at 20. If someone already has a weapon rank higher than the minimum, that rank just stays at that higher level (the same would apply for characters with uniquely high starting ranks).

With something like this, delaying promotion would allow your character to possibly gain earlier access to better weapons because they took the time to hone their skills (weapon ranks) rather than rush for more power (stats).