I own subcompacts in 9mm, .40S&W, and .45ACP. Of the three, .40S&W is by far the nastiest to shoot out of a little pistol. Were I to buy a new itty bitty pistol, I would look at 9mm or .380ACP. A small frame revo in .38spl loaded with tgt wadcutters is also an option that won't recoil like a beast.
The KT PF9 is a very light & small package. Anything in that neighborhood size or mass wise is going to kick. My neighbor's was tolerable (for me) with 115-124 non+P, but got nasty with 147gr or +P. I just don't think you will find something that size in 9mm or larger that your wife will tolerate. The SIG P938 is in the neighborhood, weight-wise, but is a lot easier to shoot well. Still kicks. The SIG P238 is the .380ACP version. Might even have to go .32ACP to get something tha small your wife will tolerate. I had to (KT P32).
I know you are not looking for a brick, but the Glock 43 single stack 9mm might be a step in the right direction. Glocks do have a noticeable (to me) effect that dampens felt recoil. Same applies to the S&W Shield.
Good luck, but the physics of the situation darned near dictate the results.