My two cents
I figured I'd give some information I haven't seen yet, or overread.
Concerning the training of the gals, it's fairly easy for some, and rather annoying for others.
First of all, くまのめいぐるみ, the bear plush doll will be essential, as it grants any gal that has one a third of any achieved EXP. Think EXP-Divider in Pokemon, just without dividing. You can get two from losing to the first gym leader twice, and I think one from losing to the rival in her house. It's also possible to buy them dirt cheap, so there's really no reason to not have at least a few.
NTR'd or presented gals from the gym leaders get a 1.5x EXP multiplier, making them easier to level.
In the first town shop, an item for 1000 called インデリ眼鏡 gives the same multiplier for any gal that wears these glasses. It also happens to be cumulative to the aforementioned effect AND also applies for the EXP granted by the plushies.
Leveling a whole team of six is rather manageable this way, though some individual gals need abnormal amounts of EXP anyway, so they'll still be a pain in the ass. The only downside is having to use two item slots on each gal for this.
Secondly, there are some alternate evolutions and form changes.
Speaking to Tamaki, the Black Co. Leader after having rescued the rival's gal, she'll allow the main gal to change into a stronger version. Erin, the white-haired girl with the sword from Tokyo Dome elite course 3rd battle can too, though I'm not sure of the conditions.
Some gals have alternate evolutions, most notably アイドル(Idol), エンジェル(Angel) and マジシャン(Magician) and ポリティ(think Eevee x Porygon).
1. Pority, which can be won from the Lottery in the fifth city, inside the gym/mall with the satellite crashed on the roof, with somewhat medium frequency. In another house, same city, fourth floor, a guy surrounded by huge sale signs sells a bunch of stones, each of which will give Pority a different evolution for a total of seven.
2. For the Angel, doing nothing will result in the regular Archangel evolution, but actively whoring her out with the cheapest option in love hotels for at least 5 times and making at least one video will make her evolve into Dark Devil instead.
3. Same deal with the Idol. Doing nothing will turn her into a Superstar, doing AV's, the most expensive option in love hotels, will make her evolve into an AV Actress instead.
4. Pretty much the same with the Magician. She'll either turn into a Holy, or Devil Magika depending on her sexual relationships.
For the rest, it's just straight up leveling them until they feel like it. At least as far as I've seen.
The album on the player pass provides some useful information, as it lists which gals have special scenes. Just as a passing note, エローナ has a ton more story related sex scenes.
For gym battles, after winning there's the option of getting a girl from the gym leader's team, or having sex with the gym leader. Choosing the first option, saving in a separate slot and then reloading a save prior to the confrontation will still leave the scene unlocked in the album, allowing you to get both the gal and the scene.
There's an option to buy effort point boosters in the dome toilet, provided you bought the password in the city before. It's タマキ様最高, copy and paste works. About thirty or so boosters are required to get full stat gain from a gal without any effort points. This way, you can maximize your girls' stats if you chose to raise them from Level 1 without needing them to ever fight themselves, if you use the plush dolls. Won't get you any affinity though, so attacks using them will stay weak.
I think that's it for now, I can't think of anything else that hasn't been said already. If there are any non-tech-support questions, I'll try my best to answer them.
