I agree with your statement, discipline is very hard raise, and the only way I was able to raise it, is when I talk to them about their chores. The chances of it raising it quite low and it's even worse if you have low charisma.
However, when I did have a high discipline for my slaves, I found is extremely easy for me to increase their skills, so IMO I think that the way to obtain discipline is balance for now, due to its ability to turn a low level slave into a god.
Edit:
If you want a fast way to increase discipline, I would first suggest that you have a high charisma so that you won't get the negative stats when you fail. In addition, you might have to utilize the scum save tactic where you save before talking to them about their chores, and reloading it to the saves if you didn't get it.
If you want to do it the legit way, you gotta rely on rng.