I think パートナーチェンジ series is my first rarest sitCD I found, the reason I listened was because of the unique premise "swapping partners." (well basically its NTR but not kinda?) It's old sitCD (2018 ish) so I don't really remember the whole story, but there was a reason why the three of them decided to swapped each partners and I cried a lil bit on vol 3 www
second is ばぶばぶ本舗 series, kinda curious about the premise—where you want to be pampered by a man when you're tired, and it turns out the club you walk into is one where they act as babysitters (you call them "papa"), and you become the baby who gets pampered by them.
Actually, there's another drama CD with the exact same with ばぶばぶ本舗 series, but they're playing as "moms". At first, I was a little confused and uncomfortable listening to it (same goes with ばぶばぶ本舗), but for some reason, it gave me insight and how unique about how the seiyuu were able to use motherly voices