9mm is a perfect self defense ammo
There is no "perfect" ammo or gun. Everything is a compromise.
--Some are very small and easily concealable, but are marginal in power and/or capacity.
--Higher capacity means larger and usually fatter (double-stack).
--A larger, more powerful cartridge either means a larger, heavier pistol or a light pistol with brutal recoil.
You will have to experiment and determine what is appropriate for you. I started small and light, and am learning how to conceal larger and heavier handguns every year.
Seven years into my CCW experiment, I am currently carrying a 4" steel S&W N-frame in the winter, and a .45 acp Commander-sized (steel or alloy framed) 1911 the remainder of the year.
FWIW, I am 54 y.o., 5'9", 152 lbs, with a spinal fusion sugery behind me. I can't run as fast as I used to, and can't fight H2H as well, so I take some comfort in carrying something I shoot well and have some confidence in.