You are correct about the electret/dynamic setting in the radio. The electret mode puts power on the audio wire to power the element. With a power mic, this voltage is going to the amplifier output in the microphone and is not helpful in any way and could possibly damage the microphone or radio or both. So with a power mic, always choose dynamic no matter what type of element it uses.
A great electret mic to use with these radios is the Uniden bc645. I use one on my Stryker 955 and get great reports. Listening on my station monitor, it is a very natural sounding mic. On sale now for $15.
Uniden BC645 4-Pin Microphone replacement for CB Radios, Comfortable Ergonomic Design, Rugged Construction, Clear Quality Sound, Built for the Professional Driver
Amazon product ASIN B008CLIB5K