Hi. I'm glad you are enjoying the game.
I know that it's really opaque right now regarding girl's feeling during training. I am planning to improve it. Think I'll add some visual indicators to make it clear whatever she enjoys it or in pain etc. Also some dialogues to explain to the player how to perform better.
But I have to add that it's actually not necessary to make slaves enjoy the process. That's the player's choice what kind of relationships he wants to build.
Regarding discipline. For the best results the severity should roughly match her misbehavior level that day. But again, there are no hard rules. You can spank a slave harder if you want her to fear you more. Different implements are just for variety.
Most of the mechanics are already in place. Now the my goal is to improve them and add content (main story, side quests, new slaves and NPCs, locations etc.