Please note that I was half-shitposting and my brain is broken w/r/t keyboards. You need to take everything I write with the understanding that it comes from someone who is this far down the rabbit hole. :)
That said: Red or brown switches will be quietest. Do not get blues.
Reds will be very light, light enough that some people have trouble with errant keypresses (particularly those who rest their fingers on the keys).
I won't lie, any mechanical keyboard will be louder than the MX Keys by default just because of key travel/bottoming out even if you're a light touch. But maybe it's acceptably loud (typing test; contrast with how loud the mouse click is). There are things you can do to make them quieter—o-rings on the keys, foam in the case, installing specifically "silent" switches—but if typing sounds make you/your wife's teeth itch that may not be enough.
If you do end up getting that particular keyboard, I would suggest the hotswap version which will allow you to change out switches without having to desolder anything.
