The game is unquestionably more intricate to navigate than it used to be, and it takes longer to complete all achievements. I think you've greatly exaggerating when you say you get OP within 20 minutes.
In part this is because there are more of them, but also in part because the game parameters have been adjusted over time. It's much harder to obtain the appeal levels, sex experience and sexual control levels necessary to fully satisfy a lot of the girls. Plus you can no longer invest your exp into buying more simulation days, so you have to chase the achievements that provide more days if you want to get more done in a single run rather than going for more repetitions. Targeting them early is a player choice.
If it was a simulationist game I could agree with you, it's pure fantasy to think one student could keep his relationships with all these girls separate and without jealousy. The idea that you gain popularity with everyone for sleeping around with their friends and classmates is porn fantasy not reality, but it does make for a fun game. It's also pure fantasy that a man could recharge so quickly that he could spend all day every day going directly from one sexual encounter to another.
I'm not sure that taking a fantasy time-machine sex game and making it more simulationist is a good idea. Introducing jealousy would undermine the core mechanic of building popularity through sex. Nor does having to repeat the early phases of the run more often help keep it entertaining. We spend exp to be able to reset our virginity multiple times a run precisely to reduce the number of runs that we are grinding out just to unlock the 'lost virginity to' achievements. We spend exp unlocking starting cash to eliminate the need to do more runs just to afford all the things that now cost lots of money (e.g. Zoe). It's quite hard at the moment to hit 90 str/stam so that's a dull run or two just grinding those out even after you've unlocked extra days and Layla's boosts if you are a completionist.
I'm pretty sure it takes a minimum of 5 runs to reach a point of completing everything except losing virginity to each girl now, and at least a couple more if you really want to do everything. I'd say that was enough. We already have content gated behind Jane love/payment and Ms Pearl cash/sex blackmail. These sorts of gates feel sufficient, to me. There are lots of other games where the game treats each run as unique, relationships are mostly exclusive and harems are not possible. I'd rather not see this one go down that path. It doesn't really fit with the porn focus that includes many combinations of threesome and now even foursome content.
Caveat: Everything I'm saying is before trying out 0.69, and I'm looking forward to it.
I didn't exaggerate, but I also didn't say you got OP in 20 minutes. "30-40 minutes" isn't an exaggeration, as I was watching the clock for IRL reasons that night. The reason there's a gap is that IMO the (already mild) grind disappears about 10-20 minutes before you become OP. I think it's OP once you no longer have to prioritize anything or make choices to get most of the girls. It loses the tactical/strategic decision making that I look for in a game. I think that's OP, especially when it happens before you have even 10% of the rewards.
My suggestion about jealousy was exactly about that 'popularity through sex' that you reference. I didn't see it as the 'core mechanic' (just two of the stats) but once you're good enough at sex and popularity, there are some girls who jump in your bed and that makes sense. But, at the same time there are a few girls you get via real or implied promises. Those girls are essential for the first run(s) when your character is a skill-free loner, but they could be harder to convince to keep things open/casual. The more casual girls wouldn't be jealous, but plausible eventual content could include making them treat the relationship more seriously without exclusivity. Do you dislike Jane's two routes?
It sounds like you're suggesting a game has to be a simulation to benefit from realism. Obviously, sometimes you're picking between them, and I'm quite happy with no boys and the only girls visible are the ones who might sleep with my character. But if you can add realism without losing anything? That's a good thing. Plausibility leads to immersion leads to everything else in the game having more impact. That suggestion was for ways to make the game more engaging as a game in a way that also removed a small plot-hole and cost us zero fantasy outcomes.
I also love that this game goes to 'full harem mode' but things like that feel even better if you feel like you earn them. I'd never want to see the more innocent girls locked away outside of exclusive runs, but making that another challenge just gives you something more fun to do mid-game, before you've beaten everything and just start repeating the game for completion's sake.
Please remember the first line of my review

My criticism is intended as constructive for a game I think is really good and could be even better. I'll keep playing future updates regardless of whether either of my suggestions are implemented.