Totally got what you are saying. I’m just saying that you are very limited on your specs because you want the thinnest possible. I’m saying that I haven’t seen ground breaking differences between mods that are basically the same size, long and skinny. I have owned the Huggett Snipe 1.22” (like the Koi). I liked it a lot, and was a touch quieter to me that some others I tried that were bigger. The Koi is well regarded at the same size, and it would be hard for me to think it could be very different in sound, but it is cheaper. Either go with price or with looks, as I don’t think one or the other is going to be massively different and make the different in whether neighbors here you.
The other piece not mentioned, that determines the quietness way more than the mod is the power/speed you are shooting. Some mods are designed to work with different levels of power (not the two I mentioned). If you really want to get quieter, stay with lighter pellets at slower speeds. For example, my Crown or Impact was significant quieter shooting 18gr at 880 vs shooting 25gr at 940. I honestly think that is where you will find your biggest difference and what will help you the most.