As far as systems are concerned...
This is a lot, but just throwing out a skeletal framework for a slave trainer system. These ideas build on each other and could spiral into a huge system. I'm aware it's solo project and I'm not a coder so take what inspiration you like and feel is reasonable.
More stats on the girls to level up than just obedience and love. More attributes will branch out to multiple training options. Each training option should have both positive and negative effect on attributes, maybe +1 +1 -1 to various stats depending on the training. Like spanking/whipping +1 obedience -1 love, +1 Pain Tolerance/masochism.
If you wanted to get more complicated you could adjust the numbers based on personality traits or kinks. Like a Masochist might get +1 love, +1 pain tolerance, -1 obedience (They like the spanking and see it as a reward for misbehaving.) Timid might get +2 obedience
Traits to level up organized by category.
Skills and traits capable of increase/decrease (Leveling and balancing these should increase brothel earnings and cause/prevent hopeless/catatonic useless mental state or sex crazed insanity)
Oral (both male and female), Vaginal, Anal, Tit job, hand job, foot job, BDSM (Top or bottom Dominatrix or pain toy), Performance (Dancer, contortionist, masturbate, use toys on self or others), Obedience, Happiness, Sanity, Lewd, Etiquette, Cleaning, Cooking, Flexible (contortionist, yoga, gymnastic performance), Religious, Persuasive, Voyer
Personality Traits
Defiant, Timid, Confident, Anxious, Spoiled, Arrogant, Treacherous, Dominant, Submissive, Studious, Perky, Hopeless, Pure, Proper, Proud, Humble, Shy
Training methods
Forced Hand/Tit/Foot job, Forced Vaginal, Forced Anal, Mouth Gag Oral, (Unlock willing Hand/Tit/Foot Job, Vaginal, Anal, Oral options) Whip, Candle wax, Anal Beads, Enema, Douche, Hot/Cold water spray, Clothes pins, push pins, Anal plug, Chastity device, Masturbate, Public Nude, Bathroom with audience, Cooking, Cleaning (multiple pic options, cleaning kitchen, cells, master bedroom, slave latrines, cleaning with tongue), cunnilingual, fingering, massage, sex toys, (pleasuring the slave) or maybe vampire hypnosis induced self pleasure, fine dining, conversation, work as bar maid, work for church, Etiquette training, Meditation, Yoga, Classy Dance, Lewd Dance.
Specific Vampire and old castle themed ideas could be wooden horse, stockade rape, the rack, bed of nails (in a coffin) feeding on the victim, torture by summoning swarms of bats/rats/spiders/snakes to swarm over a bound slave and/or burrow into vag, anus, mouth (gag?) ears, nose, bite and scratch.
(Summon Swarm could also be a combat spell)
On that note combat could have cut scenes with the attacks that lead to loss of armour/nakedness as hitpoints drop.
I could probably expand on a lot of those ideas, but lets start with if you even like any of them, then we can look more closely at what you want to use and how it could be balanced.