I dont like the idea of manual promotion neither. It sounds very very micromanagement-heavy, and so non-immersive when you consider the historical scenario.
At least it shouldn't be the only way to promote pop, instead it should be alongside the natural promotion/demotion mechanic of pops.
Think about how social mobility is achieved in the classical era: Slaves could buy their freedom with their saving, this practice was confirmed to exist in Rome and Greece, but banished in Athens. So in terms of the game, a slave pop with wealth can promote itself into the freeman class, this promotion can be a long progress, with the speed affected by laws, pop wealth, etc. Just like the current pop growth mechanics.
We knew a specific way for citizens and freeman to demote into slave historically: debt slavery. It's abolished in rome, but exist in many other classical societies. So a free pop could end up demoted into slavery because of bankruptcy - just like vicky 2 when bankrupt pop always have a sharp demotion rate. Of course you can abolish debt bondage with a law passed in senate.
This is what i hoped for: passing or repealing laws to affect the promotion/demotion rate (Augustus passed law to control the amount of slaves you can free for once, that sounds like what you would want to do late in the game), and control the dynamic pops in a grand scale. Instead we got static pops, with only micromanagement mana the only way to affect their status.
I sort of like the concept you described and it's probably not too late to add something similar. But what determines pops wealth/bankruptcy? In Vicky 2 there was a taxation system. But in Rome I don't think there is (at least not in Vicky 2 fashion). So there needs to be a well conceptualized system behind this mechanic before it's implemented.