Oh, yeah, making that a 3d object would definitely be overkill, especially since the characters the clothes are actually going to be on are 2d.
Making the characters be instantly available seems sensible, at least for the first few. Although I feel like there'd be more of a sense of progression if eventually there were "more difficult" customers that you'd unlock. Maybe via the special extra voodoo items like we were discussing before in the other game's comments. If all the characters were available right away, then you'd see each individual one pretty infrequently, so it would be harder to gauge your own progress, and once you had enough needles/touch power/whatever you do to make climaxes easier to get one of them to "max level" (getting them nude, seeing all the facial expressions, getting their clothes fully wet, whatever the metric might be so that there's nothing new to see after it's done), then you'd have enough to see that for all of them, and the game would essentially be over. Plus, any system you make would let you eventually add in the shop owner as the highest stage in the progression.
I had thought of two ways new people could show up after the initial first batch: either it's via getting new voodoo items that fit their personal tastes, like I said before. Theoretically, these people would have maybe visited the shop before and decided not to participate because a specific desire of theirs wasn't catered to by their current "stock". I feel like this is a good idea, considering that would naturally make it so that you didn't have to add a ton of variations for each customer for each new voodoo item you got. The second idea is a bit simpler: New customers just get recommended to the shop by a previous customer once they reach a specific level of satisfaction. Maybe a combination of both (for some customers), and if you fulfilled the recommendation requirement but not the correct type of voodoo equipment, the shopkeeper would give you a message like "there was a lady who came in today asking if we had ____ available in our services."
If you wanted, you could send me some of those customer sketches over discord, and I could tell you which I liked and that sort of thing. Might help you to feel more inspired about one of them, or I could suggest something entirely new. Plus I'm kinda curious about it too, lol.
But yeah, that's all for later once those other projects are done and you're working on this some more. I'm definitely going to be keeping an eye out for those when they come out.