I doubt there is enough terminal-ballistics difference, in a living target, to matter, between 9mm and .40 S&W. Staying with 9mm is what I would have advised. I gladly “down-sized,” from .40 to 9mm, in 2015, for police patrol duty, as soon as my chief OK’ed 9mm as an alternative primary duty cartridge. Shooting .40 had become painful, by my 50th birthday, in 2011. I had been using .40 S&W on duty since 2002, and during most personal time since 2006. (I do not “blame” the .40 for being the main cause of my aching hands. The .44 and .41 Mags, fired through N-Frames, too big for my K/L-sized* hands, in the Eighties, were the main problem,)
I had tended to carry .40 P229 SIGs, both on and off the clock, but when .40 started hurting. I brought my full-sized, all-steel 1911 .45 ACP from the safe, and, thankfully, found that I got right back into the groove. Yes, .45 ACP, is kinder and gentler than .40, especially when fired from an all-steel pistol. I then starting adding 9mm Glocks, in anticipation of a rumor that 9mm would soon be authorized for patrol duty.
Before good controlled-expansion JHP ammo, with bullet profiles that fed reliably, had been developed, I do believe that using bigger-bore ammo was a good idea. I believe that .40 S&W made sense, at the time of its introduction. I bought an S&W Shorty Forty, and the .40 version of the Browning High Power, in the early Nineties. Neither managed to dethrone my favored .357 Mag and .45 ACP handguns. By the late Nineties, good 9mm JHP ammo existed, which made the .40 S&W unnecessary, in my opinion.
As I saw it, shooting .40 S&W gave me terminal ballistics somewhat like that of a 185-grain 45 ACP, while being almost as obnoxious to fire, indoors, as .357 Magnum. When it became necessary to use .40 duty pistols, I used the Gen3 G22, at first, but reached a frustrating accuracy plateau, and so switched to SIG in 2004. When I was able to down-size to 9mm, in 2015, I returned to Glocks, as Gen4 fit my hands much better than Gen3, which aided accuracy.
*Actually, my hands are long, so a large-gripped gun “feels right,” but, my fingers are not long, so reaching an N-Frame trigger required that I hold the weapon in an ergonomically-compromised way, that later became known, by some firearms trainers, as the “h-grip.” This asymmetric grip directs recoils forces into the base joint of the thumb, which torques the wrist in a way that is now known to be terribly destructive. (I am not an M.D., but am married to one.)