I think you should be playing on Normal difficulty tbh. I can't even imagine playing on Hard without knowing all the ins and outs of the game.
I'd say the best fetish in the game right now is Lewd Knowledge, because after a certain lewd book from the Trademond bookstore, you can reach very high MATK values if you have high Lewd Knowledge. Exhibitionism also helps in a couple of ways, including the sex moves in combat.
Yeah, relying exclusively on Womb of Lust without any effective willpower modifiers (from fetishes/sensitivities and passives) gets you masturbation scenes, but only at the very start of the game. Here's what I do when starting out: do the hydrangea fight in the forest, using Star Knightess if needed to finish off the boss. Go to the Alchemist, do the pink lewd scene with him. Give him the spores, buy the potions, use one. Go to the jailer for Charlotte, do the pink scene with him, use another potion. Finally do the pink Hermann scene. Oh, and it's best to not be affected by Womb of Lust when starting this, because you'll get all that WP back after the first H-scene. Of course, you can do something like this with other H-scenes that don't end the day, like the guard exhib scenes in Trademond, as long as you have the Vit potions to counter the WP boost after each pink scene. Just make sure Corruption doesn't overflow and you get a Game Over.