Really love the game. It feels smooth to play and it is rewarding to slowly corrupt the girls. Even with a pretty much maxed out save, the gameplay itself is enjoyable enough for me to keep going and try to collect 100% of the girls.
On a sidenote, I feel like the progression curve is a bit too exponential for everything at once (this might sound weird but I'll explain what I mean). On base difficulty, by the time I had 10 alumnis and girls take 1-2 classes to get to 100 grading, I still didn't have much of the available building upgrades.
I don't think the difficulty scaling is at fault here, it's more the money side that's the issue.
Because of the prestige requirements, the amount of money you have to splurge to get income improvements are irrelevant by the time sponsors give you infinite money.
And sponsors are super underpowered for a very long time as you'll pretty much be at max 2 sponsors by the time you get overpowered and don't need the building ugrades anymore.
Not that getting more sponsor is fun in itself, at some point you just mechanically click the highest% photos/videos/girls and move on, which becomes a chore when you have a ton of them (although by the time you get 5, you're rich enough that you don't need to care about them anymore).
in PTA, Money is mostly used to unlock options that allows you to make more corrupted shoots that give you more money, or have girls take on shoots that they should be unconfortable with, but those options feel slightly pointless ; when the girls get corrupted enough or become alumni, they'll do them anyway. And if the girls aren't corrupted enough, they would barelly take 3 photos before bailing out, making it pointless to do the shooting or buy the upgrades by the time you have enough money to get them.
I think the game would benefit from sponsors giving you more money, but from having more valuable options to spend it as well. Making the game slightly harder to survive if you don't bribe/gift the girls and their mothers for example. You could bribe mothers to use some of the options you usually need rule breaks for, or get mother/daughter scenes. You could have the character bribe girls to take photos they wouldn't be confortable with (when they rush out saying wtf during shoots). All in all, make the game harder to progress without using money (state corruption periodic payment ?).
Next focus point would be girls statistics. Just for the fun of it, I tried to make a save where I only use fear/discipline/control options. Even after many weeks, counting home visit/training, most of the girls fear stats were still relativelly low, while their corruption was maxed out. It feels super hard to raise while going for it, but the effects were almost negligible. If that's the case, it makes a lot of sense that fear would have no impact on the game whatsoever on a normal playthough, were you can eliminate fear from a girl by kissing her 2-3 times and affection is OP.
It's not that I'd like stats to be harder to raise, as it would be a boring and grindy way to raise difficulty for no reason. Just that I believe that those stats having more impact/significance would add more variety to the gameplay. Maybe more varied girls reactions on actions based on "negative" stats? In the end, the objective would be for player actions to have more impact, whether negative or positive. Make choices matter more.
By the way, you already have a mechanic to help with that : "action strength". As for now, it only increase/decrease the impact of the actions you take, but does it make sense for a forcefull kiss to be perceived as affectionnate compared to a gentle one? Effects should be more varied depending on girls current state in order to make training decisions more impactful.
Overall, I'd finish by saying affection is too easy to raise and it's effect are slightly too strong. New girls tend to go straight from indifferent to lovey-dovey // prude to slutty.
While those were my 2 cents on ways to "improve" the game (in terms of meaningful challenge/gameplay perspective), it is absolutely arbitrary and what I said will sound like a downgrade to those who enjoy the game in it's peacefull state.
PS: Can't wait to try the headpat mod
PPS: There probably already have been talk about balance in the past, sorry if I repeated things you already considered/answered.
PPPS: Even if the game was to stay as is, which i'm sure it will not considering community's love and the time you spend on it, it's already 6/5