I tried this game and found it to be interesting. I'm not the biggest fan of how one accrue a workforce.
This game confusing to me in the start. I offer a few tips to newcomers, that I hope, can be of help.
Training the low graded girls is mostly to "correct" their stats. Should they have all too much of one and too little of another stat.
Missionary or licking seems to work best in the start as a mean to increase their stats. You don't need the lewd stat in the first area.
The stat increases seems to be based on a roll (where the grade is also a factor). Basing the results on a total of lost/gained. Meaning you'll see a lot less stat increases in training, if there's nothing to substract from. Two stats gained and one lost is still a positive. There is still a chance to see increases (besides loyality/mental), but they are less common.
You are often better off sending them to work, their stats will (to a certain degree) grow without the negatives.
You should unlock the promotion coach first (so you can earn trust).
Unlocking a new area will help a lot on your income.
Their increase in stamina regen is very significant with the endurance training, they'll be able to work almost 100% of the time. And I mean almost a 100%. Depending on the stats/grades of the girls and the area in which they work, you'll probably not even get to see them rest ones in their career... which is kind of sad.
The traits really do matter a lot.
The Monstrous Bitch, Slutty III, and Righteous Boobs traits are very powerful to have on your girls.
Sometimes you are better off rushing, to get the girl be an assistant and give the other girls her important trait. Waiting for her to wither and then make her an assistant can sometimes cost more in the end. Having your low–mid tier girls be slutty (or righteous about their big boobs) is a huge advantage and depending on your situation, it can double your profit margin.
What can halt your progress in the mid–game is aiming for too highly graded and thus much more expensive girls. You'll really not be able, to utilize much of their extra stats, until you unlock the last areas. Girls of grade C to B+ should do just as fine.
The last building upgrade takes priority; you need it, to have enough trust, to get the last areas.
Cheers and kind regards from Hajtand.