So, quick review, feedback, and suggestions.
Overall, game's a great framework, love the loop, played it quite a few times, so let that be the baseline for my criticisms.
There isn't enough clarity on what stats actually do for you or a girl outside of openness. I feel that popularity increases tips, maybe? You've got two different types of work experience, Experience and Work Proficiency and I have no idea what either really does in game, so I just ignore them. I can't tell whether charisma is affecting training or not, but it doesn't seem to matter because early game I can just dump time into training using books to 90+ charisma almost immediately. Popularity seems important but there's no way to affect it.
This leads to the problem where the only start I'd want to do is the 20k start, because strength doesn't matter (yet) and charisma can be capped too quickly and only works on some training (with no transparency as which it does and does not). 10 charisma is just not worth 5k.
The other problem is that girls training really just comes down to 'pick highest stat's training' and that's not terribly deep or interesting on your 7th or 8th playthrough.
How would I resolve this?
Well, the most important interaction with girls is that 0-10 stage of training them before they become self-reliant. I don't think I'd want to change this inherently because no one really wants to train a girl past a certain point. So how do we improve this without harming the fundamental gameplay we enjoy?
Rather than have charisma maybe affect all training, have each stat affect a different attitude of training.
So, for example, massage skill could be replaced with something reflecting more hands-on-ness--maybe call it Rakishness, reflecting your flirtatiousness. This could be paired with Lewdness-based training, because your ability to flirt with them might encourage them to be more flirtatious. This in turn can be used to increase the rate at which girls gain openness passively after the 10 stage, or gain work experience, potentially through the spa or bathrooms.
Charisma can be reworked to Salesmanship. Obviously this would pair with Boastfulness, and would also be used to affect costs of supplies, get better returns on supplies (which can be used to reduce the dominance of the bar early game for other builds) and could be used as a stat governing the ability to train Popularity, which would increase girls' costs but also increase girls' returns.
Strength could be changed to Intimidation for both you and the girls, and paired with Obedience for that style of training. I'd want to see what the expanded bouncer play actually ends up being like before commenting but the idea of having a stat which is a defensive trait in terms of running your club seems interesting. It'd also depend what other management style stuff you bring in but this stat already has the promise of affecting strategy outside the training layer so this is really more about bringing it into the training layer.
This basically changes the early-mid game a bit, such that you'd consider a girl's stats and how they work within the framework of your own, as well as making the ancillary systems matter (or at least seem to matter more)
Overall though, enjoyable game. It just needs a biiiiiit more depth in the management layer, some way to make stats feel like they matter more and then it'll be something I'll be playing again and again. More than I already do.
EDIT: Oh, also, Quality of Life addition: In the training menu, if a girl is unavailable to be talked to because they're someone else, have something mention where they are. Possibly the button that's grey'd out now just says the location, 'In Gym' for example.