It is not really recommended to use frozen chicken in the slow cooker, since there’s risk of bacteria contaminating the meat before it reaches a safe temperature.
If you prefer a tender, juicier chicken, I recommend using chicken thighs instead of chicken breast. The meat from the thighs is much more juicier than that from the breast and contains more flavor as well.
When using soy sauce, make sure you are using a high quality soy sauce that has a lower sodium grade. This way you are able to control the amount of sodium intake and get a tastier flavor as well.
When prepping the chicken, if you find you’d like a little extra smoky flavor, you can always add 1 tsp of liquid smoke to boost up the flavors.