My worry about adding other ways to get punisher effectiveness percentage is that the requirement for T4 breaks would have to be increased (or the gains nerfed) in order to keep the same level of difficulty, and that would have the potential to make the game more grindy instead of less. I want the sinful moves to still be the main way that punisher effectiveness percentage is increased, because it presents the player with a choice. You can stop the Chosen from using the sinful moves (by using unupgraded Commanders against Detonate-users, or by surrounding Slaughter- and Striptease-users before their allies) in order to avoid their powerful effects, but this puts the targets' corruption on hold. And the more the sinful moves hurt you, the more progress you gain in their users' corruption, because both the effects and the corruption speed depend on how high the target's circumstance damage is.
The problem is that the main difficulty resides in the grind itself and lack of information. Reducing the grind is going to make it easier, this is why your Easy Mode has a cheat to unlock the achievements which includes knocking off 75% of the requirement to achieve T4 breaks in the first place right?
Because hitting 1000% is quite grindy, even if you use cheat engine to modify values so you can achieve the requirements for high level fantasize, striptease, or slaughter corruption procs. Once you understand how those procs work, because the game does not do a good job explaining them, the difficulty goes away and the grind sets in. It becomes easy to control the party to keep them locked down but it becomes a problem of herding them to do what you want them to enough times over enough battles.
Though hell, half of my main suggestion involves basically throwing on Bad Ends for the Heroines at double what is needed for a T4 break and throwing things in to ease the grind a bit and to give a bit more control. Also depending on how you look at it, the suggestion also in a sense makes things harder because you need to manage the corruption not just because some of the adaptions can be annoying* but because if you want a Forsaken that would not be effectively useless (You have exposure weaken Forsaken, so maxing out exposure triggering the Unchosen Bad End would result in a very weak Forsaken), be shall we say uninteresting (a Doll/Vessel of the Demon Lord may be able to emulate implanted personalities but its still an empty shell the player can choose to inhabit and training would likely not be as fun for some), or you simply do not get a Forsaken you can train and use in combat (Broodmother or Bio-Power Cell Bad Ends).
Though honestly one thing you could do if you wanted to raise difficulty while you lower grind would be rather simple: give the Magical Girls the ability to actually help each other when they are taken captive.
Lets use Hypnosis as an example. Conceptually Hypnosis grows in strength through repeated use on the target, so the Hypnosis commander raising Hypnosis percent every turn it holds a Magical Girl Captive makes sense. The counter to this would naturally be that if both Magical Girls are not surrounded and not a hated enemy of the one captive they would blast an escape route for their friend to escape from the brainwash ball that is the commander. Sure that would momentarily slow down the elimination of demons and thralls, but that is Magical Girls for you. They will often times put their friends and teammates over eliminating minions when possible as each combat capable magical girl is going to make the process faster overall.
Same thing with Drain, the commander being focused on a single target makes it open to a combined counter to free the one being drained. Same with impregnation. Parasitism could be countered with a combined spell to force the commander out of the costume.
This is actually why most magical girl porn games feature the Magical Girls either not working together, being solo to begin with, or the villain attacks in multiple places to split them up... which could be an option for Forsaken, sending them out on simultanious attacks following their own judgement according to personality thus allowing you to isolate one magical girl to focus your torment on at the cost of not being able to inflict breaks on the other magical girls.
*(I avoid causing slaughter altogether because I am playing to see Magical Girls get corrupted into lewd messes, striptease is a non issue because it happens most when you are setting up tempts and a tempt is a tempt each one is going to reduce the minimum needed for the next and the final battle regardless of the damage done during it)